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NAME
DBI - Database independent interface for Perl
SYNOPSIS
use DBI;
@driver_names = DBI->available_drivers;
@data_sources = DBI->data_sources($driver_name, \%attr);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement);
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_arrayref($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rc = $sth->bind_param($p_num, $bind_value);
$rc = $sth->bind_param($p_num, $bind_value, $bind_type);
$rc = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rc = $sth->bind_param_array($p_num, $bind_values, \%attr);
$rv = $sth->execute_array(\%attr);
$rv = $sth->execute_array(\%attr, @bind_values);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work;
$rc = $dbh->commit;
$rc = $dbh->rollback;
$quoted_string = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
$rc = $dbh->disconnect;
*This synopsis above only lists the major methods.*
GETTING HELP
If you have questions about DBI, you can get help from the
*dbi-users@perl.org* mailing list. You can get help on subscribing and
using the list by emailing:
dbi-users-help@perl.org
Also worth a visit is the DBI home page at:
http://dbi.perl.org/
Before asking any questions, reread this document, consult the archives
and read the DBI FAQ. The archives are listed at the end of this
document and on the DBI home page. The FAQ is installed as a DBI::FAQ
module so you can read it by executing "perldoc DBI::FAQ".
To help you make the best use of the dbi-users mailing list, and any
other lists or forums you may use, I strongly recommend that you read
"How To Ask Questions The Smart Way" by Eric Raymond:
http://www.tuxedo.org/~esr/faqs/smart-questions.html
This document often uses terms like *references*, *objects*, *methods*.
If you're not familar with those terms then it would be a good idea to
read at least the following perl manuals first: perlreftut, perldsc,
perllol, and perlboot.
Please note that Tim Bunce does not maintain the mailing lists or the
web page (generous volunteers do that). So please don't send mail
directly to him; he just doesn't have the time to answer questions
personally. The *dbi-users* mailing list has lots of experienced people
who should be able to help you if you need it. If you do email Tim he's
very likely to just forward it to the mailing list.
NOTES
This is the DBI specification that corresponds to the DBI version 1.34
("$Date: 2003/02/28 17:50:06 $").
The DBI is evolving at a steady pace, so it's good to check that you
have the latest copy.
The significant user-visible changes in each release are documented in
the DBI::Changes module so you can read them by executing "perldoc
DBI::Changes".
Some DBI changes require changes in the drivers, but the drivers can
take some time to catch up. Newer versions of the DBI have added
features that may not yet be supported by the drivers you use. Talk to
the authors of your drivers if you need a new feature that's not yet
supported.
Features added after DBI 1.21 (February 2002) are marked in the text
with the version number of the DBI release they first appeared in.
Extensions to the DBI API often use the "DBIx::*" namespace. See "Naming
Conventions and Name Space" and:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
http://search.cpan.org/search?query=DBI&mode=all
DESCRIPTION
The DBI is a database access module for the Perl programming language.
It defines a set of methods, variables, and conventions that provide a
consistent database interface, independent of the actual database being
used.
It is important to remember that the DBI is just an interface. The DBI
is a layer of "glue" between an application and one or more database
*driver* modules. It is the driver modules which do most of the real
work. The DBI provides a standard interface and framework for the
drivers to operate within.
Architecture of a DBI Application
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the call
interface and variables for Perl scripts to use. The API is implemented
by the Perl DBI extension.
The DBI "dispatches" the method calls to the appropriate driver for
actual execution. The DBI is also responsible for the dynamic loading of
drivers, error checking and handling, providing default implementations
for methods, and many other non-database specific duties.
Each driver contains implementations of the DBI methods using the
private interface functions of the corresponding database engine. Only
authors of sophisticated/multi-database applications or generic library
functions need be concerned with drivers.
Notation and Conventions
The following conventions are used in this document:
$dbh Database handle object
$sth Statement handle object
$drh Driver handle object (rarely seen or used in applications)
$h Any of the handle types above ($dbh, $sth, or $drh)
$rc General Return Code (boolean: true=ok, false=error)
$rv General Return Value (typically an integer)
@ary List of values returned from the database, typically a row of data
$rows Number of rows processed (if available, else -1)
$fh A filehandle
undef NULL values are represented by undefined values in Perl
\%attr Reference to a hash of attribute values passed to methods
Note that Perl will automatically destroy database and statement handle
objects if all references to them are deleted.
Outline Usage
To use DBI, first you need to load the DBI module:
use DBI;
use strict;
(The "use strict;" isn't required but is strongly recommended.)
Then you need to "connect" to your data source and get a *handle* for
that connection:
$dbh = DBI->connect($dsn, $user, $password,
{ RaiseError => 1, AutoCommit => 0 });
Since connecting can be expensive, you generally just connect at the
start of your program and disconnect at the end.
Explicitly defining the required "AutoCommit" behaviour is strongly
recommended and may become mandatory in a later version. This determines
whether changes are automatically committed to the database when
executed, or need to be explicitly committed later.
The DBI allows an application to "prepare" statements for later
execution. A prepared statement is identified by a statement handle held
in a Perl variable. We'll call the Perl variable $sth in our examples.
The typical method call sequence for a "SELECT" statement is:
prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a *non*-"SELECT" statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chomp;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The "do()" method can be used for non repeated *non*-"SELECT" statement
(or with drivers that don't support placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when "AutoCommit" is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you should
"disconnect" from it:
$dbh->disconnect;
General Interface Rules & Caveats
The DBI does not have a concept of a "current session". Every session
has a handle object (i.e., a $dbh) returned from the "connect" method.
That handle object is used to invoke database related methods.
Most data is returned to the Perl script as strings. (Null values are
returned as "undef".) This allows arbitrary precision numeric data to be
handled without loss of accuracy. Beware that Perl may not preserve the
same accuracy when the string is used as a number.
Dates and times are returned as character strings in the current default
format of the corresponding database engine. Time zone effects are
database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary
data to and from the driver without change. It is up to the driver
implementors to decide how they wish to handle such binary data.
Most databases that understand multiple character sets have a default
global charset. Text stored in the database is, or should be, stored in
that charset; if not, then that's the fault of either the database or
the application that inserted the data. When text is fetched it should
be automatically converted to the charset of the client, presumably
based on the locale. If a driver needs to set a flag to get that
behaviour, then it should do so; it should not require the application
to do that.
Multiple SQL statements may not be combined in a single statement handle
($sth), although some databases and drivers do support this (notably
Sybase and SQL Server).
Non-sequential record reads are not supported in this version of the
DBI. In other words, records can only be fetched in the order that the
database returned them, and once fetched they are forgotten.
Positioned updates and deletes are not directly supported by the DBI.
See the description of the "CursorName" attribute for an alternative.
Individual driver implementors are free to provide any private functions
and/or handle attributes that they feel are useful. Private driver
functions can be invoked using the DBI "func()" method. Private driver
attributes are accessed just like standard attributes.
Many methods have an optional "\%attr" parameter which can be used to
pass information to the driver implementing the method. Except where
specifically documented, the "\%attr" parameter can only be used to pass
driver specific hints. In general, you can ignore "\%attr" parameters or
pass it as "undef".
Naming Conventions and Name Space
The DBI package and all packages below it ("DBI::*") are reserved for
use by the DBI. Extensions and related modules use the "DBIx::"
namespace (see "http://www.perl.com/CPAN/modules/by-module/DBIx/").
Package names beginning with "DBD::" are reserved for use by DBI
database drivers. All environment variables used by the DBI or by
individual DBDs begin with ""DBI_"" or ""DBD_"".
The letter case used for attribute names is significant and plays an
important part in the portability of DBI scripts. The case of the
attribute name is used to signify who defined the meaning of that name
and its values.
Case of name Has a meaning defined by
------------ ------------------------
UPPER_CASE Standards, e.g., X/Open, ISO SQL92 etc (portable)
MixedCase DBI API (portable), underscores are not used.
lower_case Driver or database engine specific (non-portable)
It is of the utmost importance that Driver developers only use lowercase
attribute names when defining private attributes. Private attribute
names must be prefixed with the driver name or suitable abbreviation
(e.g., ""ora_"" for Oracle, ""ing_"" for Ingres, etc).
SQL - A Query Language
Most DBI drivers require applications to use a dialect of SQL
(Structured Query Language) to interact with the database engine. The
"SQL Standards Reference Information" section provides links to useful
information about SQL.
The DBI itself does not mandate or require any particular language to be
used; it is language independent. In ODBC terms, the DBI is in
"pass-thru" mode, although individual drivers might not be. The only
requirement is that queries and other statements must be expressed as a
single string of characters passed as the first argument to the
"prepare" or "do" methods.
For an interesting diversion on the *real* history of RDBMS and SQL,
from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the "And the rest" and "Intergalactic dataspeak" links for the
SQL history.
Placeholders and Bind Values
Some drivers support placeholders and bind values. *Placeholders*, also
called parameter markers, are used to indicate values in a database
statement that will be supplied later, before the prepared statement is
executed. For example, an application might use the following to insert
a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The "?" characters are the placeholders. The association of actual
values with placeholders is known as *binding*, and the values are
referred to as *bind values*.
When using placeholders with the SQL "LIKE" qualifier, you must remember
that the placeholder substitutes for the whole string. So you should use
""... LIKE ? ..."" and include any wildcard characters in the value that
you bind to the placeholder.
Null Values
Undefined values, or "undef", can be used to indicate null values.
However, care must be taken in the particular case of trying to use null
values to qualify a "SELECT" statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an "undef" (NULL) to the placeholder will *not* select rows
which have a NULL "product_code"! Refer to the SQL manual for your
database engine or any SQL book for the reasons for this. To explicitly
select NULLs you have to say ""WHERE product_code IS NULL"" and to make
that general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders. Sadly, that more general
syntax doesn't work for Sybase and MS SQL Server. However on those two
servers the original ""product_code = ?"" syntax works for binding
nulls.
Performance
Without using placeholders, the insert statement shown previously would
have to contain the literal values to be inserted and would have to be
re-prepared and re-executed for each row. With placeholders, the insert
statement only needs to be prepared once. The bind values for each row
can be given to the "execute" method each time it's called. By avoiding
the need to re-prepare the statement for each row, the application
typically runs many times faster. Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) or die $dbh->errstr;
while (<>) {
chomp;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) or die $dbh->errstr;
}
$dbh->commit or die $dbh->errstr;
See "execute" and "bind_param" for more details.
The "q{...}" style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
"qq{...}" operator if you want to interpolate variables into the string.
See "Quote and Quote-like Operators" in perlop for more details.
See also the "bind_column" method, which is used to associate Perl
variables with the output columns of a "SELECT" statement.
THE DBI PACKAGE AND CLASS
In this section, we cover the DBI class methods, utility functions, and
the dynamic attributes associated with generic DBI handles.
DBI Constants
Constants representing the values of the SQL standard types can be
imported individually by name, or all together by importing the special
":sql_types" tag.
The names and values of all the defined SQL standard types can be
produced like this:
foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
printf "%s=%d\n", $_, &{"DBI::$_"};
}
These constants are defined by SQL/CLI, ODBC or both. "SQL_BIGINT" is
(currently) omitted, because SQL/CLI and ODBC provide conflicting codes.
See the "type_info", "type_info_all", and "bind_param" methods for
possible uses.
Note that just because the DBI defines a named constant for a given data
type doesn't mean that drivers will support that data type.
DBI Class Methods
The following methods are provided by the DBI class:
"connect"
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the requested
$data_source. Returns a database handle object if the connection
succeeds. Use "$dbh->disconnect" to terminate the connection.
If the connect fails (see below), it returns "undef" and sets both
$DBI::err and $DBI::errstr. (It does *not* set $!, etc.) You should
generally test the return status of "connect" and "print
$DBI::errstr" if it has failed.
Multiple simultaneous connections to multiple databases through
multiple drivers can be made via the DBI. Simply make one "connect"
call for each database and keep a copy of each returned database
handle.
The $data_source value must begin with ""dbi:"*driver_name*":"". The
*driver_name* specifies the driver that will be used to make the
connection. (Letter case is significant.)
As a convenience, if the $data_source parameter is undefined or
empty, the DBI will substitute the value of the environment variable
"DBI_DSN". If just the *driver_name* part is empty (i.e., the
$data_source prefix is ""dbi::""), the environment variable
"DBI_DRIVER" is used. If neither variable is set, then "connect"
dies.
Examples of $data_source values are:
dbi:DriverName:database_name
dbi:DriverName:database_name@hostname:port
dbi:DriverName:database=database_name;host=hostname;port=port
There is *no standard* for the text following the driver name. Each
driver is free to use whatever syntax it wants. The only requirement
the DBI makes is that all the information is supplied in a single
string. You must consult the documentation for the drivers you are
using for a description of the syntax they require. (Where a driver
author needs to define a syntax for the $data_source, it is
recommended that they follow the ODBC style, shown in the last
example above.)
If the environment variable "DBI_AUTOPROXY" is defined (and the
driver in $data_source is not ""Proxy"") then the connect request
will automatically be changed to:
$ENV{DBI_AUTOPROXY};dsn=$data_source
"DBI_AUTOPROXY" is typically set as
""dbi:Proxy:hostname=...;port=..."". If $ENV{DBI_AUTOPROXY} doesn't
begin with '"dbi:"' then "dbi:Proxy:" will be prepended to it first.
See the DBD::Proxy documentation for more details.
If $username or $password are undefined (rather than just empty),
then the DBI will substitute the values of the "DBI_USER" and
"DBI_PASS" environment variables, respectively. The DBI will warn if
the environment variables are not defined. However, the everyday use
of these environment variables is not recommended for security
reasons. The mechanism is primarily intended to simplify testing.
"DBI->connect" automatically installs the driver if it has not been
installed yet. Driver installation either returns a valid driver
handle, or it *dies* with an error message that includes the string
""install_driver"" and the underlying problem. So "DBI->connect"
will die on a driver installation failure and will only return
"undef" on a connect failure, in which case $DBI::errstr will hold
the error message.
The $data_source argument (with the ""dbi:...:"" prefix removed) and
the $username and $password arguments are then passed to the driver
for processing. The DBI does not define any interpretation for the
contents of these fields. The driver is free to interpret the
$data_source, $username, and $password fields in any way, and supply
whatever defaults are appropriate for the engine being accessed.
(Oracle, for example, uses the ORACLE_SID and TWO_TASK environment
variables if no $data_source is specified.)
The "AutoCommit" and "PrintError" attributes for each connection
default to "on". (See "AutoCommit" and "PrintError" for more
information.) However, it is strongly recommended that you
explicitly define "AutoCommit" rather than rely on the default.
Future versions of the DBI may issue a warning if "AutoCommit" is
not explicitly defined.
The "\%attr" parameter can be used to alter the default settings of
"PrintError", "RaiseError", "AutoCommit", and other attributes. For
example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
You can also define connection attribute values within the
$data_source parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence
over any conflicting values specified via the "\%attr" parameter to
"connect".
The "dbi_connect_method" attribute can be used to specify which
driver method should be called to establish the connection. The only
useful values are 'connect', 'connect_cached', or some specialized
case like 'Apache::DBI::connect' (which is automatically the default
when running within Apache).
Where possible, each session ($dbh) is independent from the
transactions in other sessions. This is useful when you need to hold
cursors open across transactions--for example, if you use one
session for your long lifespan cursors (typically read-only) and
another for your short update transactions.
For compatibility with old DBI scripts, the driver can be specified
by passing its name as the fourth argument to "connect" (instead of
"\%attr"):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this "old-style" form of "connect", the $data_source should not
start with ""dbi:driver_name:"". (If it does, the embedded
driver_name will be ignored). Also note that in this older form of
"connect", the "$dbh->{AutoCommit}" attribute is *undefined*, the
"$dbh->{PrintError}" attribute is off, and the old "DBI_DBNAME"
environment variable is checked if "DBI_DSN" is not defined. Beware
that this "old-style" "connect" will be withdrawn in a future
version of DBI.
"connect_cached"
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
"connect_cached" is like "connect", except that the database handle
returned is also stored in a hash associated with the given
parameters. If another call is made to "connect_cached" with the
same parameter values, then the corresponding cached $dbh will be
returned if it is still valid. The cached database handle is
replaced with a new connection if it has been disconnected or if the
"ping" method fails.
Note that the behaviour of this method differs in several respects
from the behaviour of persistent connections implemented by
Apache::DBI.
Caching can be useful in some applications, but it can also cause
problems and should be used with care. The exact behaviour of this
method is liable to change, so if you intend to use it in any
production applications you should discuss your needs on the
*dbi-users* mailing list.
The cache can be accessed (and cleared) via the "CachedKids"
attribute.
"available_drivers"
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching for "DBD::*"
modules through the directories in @INC. By default, a warning is
given if some drivers are hidden by others of the same name in
earlier directories. Passing a true value for $quiet will inhibit
the warning.
"data_sources"
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of all data sources (databases) available via the
named driver. If $driver is empty or "undef", then the value of the
"DBI_DRIVER" environment variable is used.
The driver will be loaded if it hasn't been already. Note that if
the driver loading fails then it *dies* with an error message that
includes the string ""install_driver"" and the underlying problem.
Data sources are returned in a form suitable for passing to the
"connect" method (that is, they will include the ""dbi:$driver:""
prefix).
Note that many drivers have no way of knowing what data sources
might be available for it. These drivers return an empty or
incomplete list or may require driver-specific attributes, such as a
connected database handle, to be supplied.
"trace"
DBI->trace($trace_level)
DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles using the
"trace" DBI class method. To enable trace information for a specific
handle, use the similar "$h->trace" method described elsewhere.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
3 - As above, adding some high-level information from the driver
and some internal information from the DBI.
4 - As above, adding more detailed information from the driver.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels 3
and above (up to 9) are best reserved for investigating a specific
problem, when you need to see "inside" the driver and DBI.
The trace output is detailed and typically very useful. Much of the
trace output is formatted using the "neat" function, so strings in
the trace output may be edited and truncated.
Initially trace output is written to "STDERR". If $trace_filename is
specified and can be opened in append mode then all trace output
(including that from other handles) is redirected to that file. A
warning is generated is the file can't be opened. Further calls to
"trace" without a $trace_filename do not alter where the trace
output is sent. If $trace_filename is undefined, then trace output
is sent to "STDERR" and the previous trace file is closed. The
"trace" method returns the *previous* tracelevel.
See also the "$h->trace" and "$h->trace_msg" methods and the
"DEBUGGING" section for information about the "DBI_TRACE"
environment variable.
DBI Utility Functions
In addition to the methods listed in the previous section, the DBI
package also provides these utility functions:
"neat"
$str = DBI::neat($value, $maxlen);
Return a string containing a neat (and tidy) representation of the
supplied value.
Strings will be quoted, although internal quotes will *not* be
escaped. Values known to be numeric will be unquoted. Undefined
(NULL) values will be shown as "undef" (without quotes). Unprintable
characters will be replaced by dot (.).
For result strings longer than $maxlen the result string will be
truncated to "$maxlen-4" and ""...'"" will be appended. If $maxlen
is 0 or "undef", it defaults to $DBI::neat_maxlen which, in turn,
defaults to 400.
This function is designed to format values for human consumption. It
is used internally by the DBI for "trace" output. It should
typically *not* be used for formatting values for database use. (See
also "quote".)
"neat_list"
$str = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls "DBI::neat" on each element of the list and returns a string
containing the results joined with $field_sep. $field_sep defaults
to ", ".
"looks_like_number"
@bool = DBI::looks_like_number(@array);
Returns true for each element that looks like a number. Returns
false for each element that does not look like a number. Returns
"undef" for each element that is undefined or empty.
"hash"
$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to the contents
of $buffer. The $type parameter selects which kind of hash algorithm
should be used.
For the technically curious, type 0 (which is the default if $type
isn't specified) is based on the Perl 5.1 hash except that the value
is forced to be negative (for obscure historical reasons). Type 1 is
the better "Fowler / Noll / Vo" (FNV) hash. See
http://www.isthe.com/chongo/tech/comp/fnv/ for more information.
Both types are implemented in C and are very fast.
This function doesn't have much to do with databases, except that it
can be handy to store hash values in a database.
DBI Dynamic Attributes
Dynamic attributes are always associated with the *last handle used*
(that handle is represented by $h in the descriptions below).
Where an attribute is equivalent to a method call, then refer to the
method call for all related documentation.
Warning: these attributes are provided as a convenience but they do have
limitations. Specifically, they have a short lifespan: because they are
associated with the last handle used, they should only be used
*immediately* after calling the method that "sets" them. If in any
doubt, use the corresponding method call.
$DBI::err
Equivalent to "$h->err".
$DBI::errstr
Equivalent to "$h->errstr".
$DBI::state
Equivalent to "$h->state".
$DBI::rows
Equivalent to "$h->rows". Please refer to the documentation for the
"rows" method.
$DBI::lasth
Returns the DBI object handle used for the most recent DBI method
call. If the last DBI method call was a DESTROY then $DBI::lasth
will return the handle of the parent of the destroyed handle, if
there is one.
METHODS COMMON TO ALL HANDLES
The following methods can be used by all types of DBI handles.
"err"
$rv = $h->err;
Returns the *native* database engine error code from the last driver
method called. The code is typically an integer but you should not
assume that.
The DBI resets $h->err to undef before most DBI method calls, so the
value only has a short lifespan. Also, most drivers share the same
error variables across all their handles, so calling a method on one
handle will typically reset the error on all the other handles that
are children of that driver.
If you need to test for individual errors *and* have your program be
portable to different database engines, then you'll need to
determine what the corresponding error codes are for all those
engines and test for all of them.
"errstr"
$str = $h->errstr;
Returns the native database engine error message from the last
driver method called. This has the same lifespan issues as the "err"
method described above.
"state"
$str = $h->state;
Returns an error code in the standard SQLSTATE five character
format. Note that the specific success code 00000 is translated to
'' (false). If the driver does not support SQLSTATE (and most
don't), then state will return "S1000" (General Error) for all
errors.
The driver is free to return any value via "state", e.g., warning
codes, even if it has not declared an error by returning a true
value via the "err" method described above.
"set_err"
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state, $method);
$rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the "err", "errstr", and "state" values for the handle. This
will trigger the normal DBI error handling mechanisms, such as
"RaiseError" and "HandleError", if they are enabled. This method is
typically only used by DBI drivers and DBI subclasses.
The $method parameter provides an alternate method name, instead of
the fairly unhelpful '"set_err"', for the "RaiseError"/"PrintError"
error string.
The "set_err" method normally returns undef. The $rv parameter
provides an alternate return value. The "HandleError" subroutine can
access and alter this value.
"trace"
$h->trace($trace_level);
$h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific handle (and any
future children of that handle) by setting the trace level using the
"trace" method.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels 3
and above (up to 9) are best reserved for investigating a specific
problem, when you need to see "inside" the driver and DBI. Set
$trace_level to 0 to disable the trace.
The trace output is detailed and typically very useful. Much of the
trace output is formatted using the "neat" function, so strings in
the trace output may be edited and truncated.
Initially, trace output is written to "STDERR". If $trace_filename
is specified, then the file is opened in append mode and *all* trace
output (including that from other handles) is redirected to that
file. Further calls to trace without a $trace_filename do not alter
where the trace output is sent. If $trace_filename is undefined,
then trace output is sent to "STDERR" and the previous trace file is
closed.
See also the "DBI->trace" method, the "$h->{TraceLevel}" attribute,
and "DEBUGGING" for information about the "DBI_TRACE" environment
variable.
"trace_msg"
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is enabled for $h or
for the DBI as a whole. Can also be called as
"DBI->trace_msg($msg)". See "trace".
If $min_level is defined, then the message is output only if the
trace level is equal to or greater than that level. $min_level
defaults to 1.
"func"
$h->func(@func_arguments, $func_name) or die ...;
The "func" method can be used to call private non-standard and
non-portable methods implemented by the driver. Note that the
function name is given as the *last* argument.
It's also important to note that the func() method does not clear a
previous error ($DBI::err etc.) and it does not trigger automatic
error detection (RaiseError etc.) so you must check the return
status and/or $h->err to detect errors.
(This method is not directly related to calling stored procedures.
Calling stored procedures is currently not defined by the DBI. Some
drivers, such as DBD::Oracle, support it in non-portable ways. See
driver documentation for more details.)
See also "install_method" for how you can avoid needing to use
func() and gain.
"can"
$is_implemented = $h->can($method_name);
Returns true if $method_name is implemented by the driver or a
default method is provided by the DBI.
ATTRIBUTES COMMON TO ALL HANDLES
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is, the value of an
inherited attribute in a newly created statement handle is the same as
the value in the parent database handle. Changes to attributes in the
new statement handle do not affect the parent database handle and
changes to the database handle do not affect existing statement handles,
only future ones.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver specific attributes (which all have names
starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
"Warn" (boolean, inherited)
The "Warn" attribute enables useful warnings for certain bad
practices. Enabled by default. Some emulation layers, especially
those for Perl 4 interfaces, disable warnings. Since warnings are
generated using the Perl "warn" function, they can be intercepted
using the Perl $SIG{__WARN__} hook.
"Active" (boolean, read-only)
The "Active" attribute is true if the handle object is "active".
This is rarely used in applications. The exact meaning of active is
somewhat vague at the moment. For a database handle it typically
means that the handle is connected to a database ("$dbh->disconnect"
sets "Active" off). For a statement handle it typically means that
the handle is a "SELECT" that may have more data to fetch. (Fetching
all the data or calling "$sth->finish" sets "Active" off.)
"Kids" (integer, read-only)
For a driver handle, "Kids" is the number of currently existing
database handles that were created from that driver handle. For a
database handle, "Kids" is the number of currently existing
statement handles that were created from that database handle. For a
statement handle, the value is zero.
"ActiveKids" (integer, read-only)
Like "Kids", but only counting those that are "Active" (as above).
"CachedKids" (hash ref)
For a database handle, "CachedKids" returns a reference to the cache
(hash) of statement handles created by the "prepare_cached" method.
For a driver handle, returns a reference to the cache (hash) of
database handles created by the "connect_cached" method.
"CompatMode" (boolean, inherited)
The "CompatMode" attribute is used by emulation layers (such as
Oraperl) to enable compatible behaviour in the underlying driver
(e.g., DBD::Oracle) for this handle. Not normally set by application
code.
"InactiveDestroy" (boolean)
The "InactiveDestroy" attribute can be used to disable the *database
engine* related effect of DESTROYing a handle (which would normally
close a prepared statement or disconnect from the database etc).
For a database handle, this attribute does not disable an *explicit*
call to the disconnect method, only the implicit call from DESTROY.
The default value, false, means that a handle will be automatically
destroyed when it passes out of scope. A true value disables
automatic destruction. (Think of the name as meaning 'inactive the
DESTROY method'.)
This attribute is specifically designed for use in Unix applications
that "fork" child processes. Either the parent or the child process,
but not both, should set "InactiveDestroy" on all their shared
handles. Note that some databases, including Oracle, don't support
passing a database connection across a fork.
"PrintError" (boolean, inherited)
The "PrintError" attribute can be used to force errors to generate
warnings (using "warn") in addition to returning error codes in the
normal way. When set "on", any method which results in an error
occuring will cause the DBI to effectively do a "warn("$class
$method failed: $DBI::errstr")" where $class is the driver class and
$method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, "DBI->connect" sets "PrintError" "on".
If desired, the warnings can be caught and processed using a
$SIG{__WARN__} handler or modules like CGI::Carp and CGI::ErrorWrap.
"RaiseError" (boolean, inherited)
The "RaiseError" attribute can be used to force errors to raise
exceptions rather than simply return error codes in the normal way.
It is "off" by default. When set "on", any method which results in
an error will cause the DBI to effectively do a "die("$class $method
failed: $DBI::errstr")", where $class is the driver class and
$method is the name of the method that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn "RaiseError" on then you'd normally turn "PrintError"
off. If "PrintError" is also on, then the "PrintError" is done first
(naturally).
Typically "RaiseError" is used in conjunction with "eval { ... }" to
catch the exception that's been thrown and followed by an "if ($@) {
... }" block to handle the caught exception. In that eval block the
$DBI::lasth variable can be useful for diagnosis and reporting. For
example, $DBI::lasth->{Type} and $DBI::lasth->{Statement}.
If you want to temporarily turn "RaiseError" off (inside a library
function that is likely to fail, for example), the recommended way
is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be restored by
Perl, regardless of how the block is exited. The same logic applies
to other attributes, including "PrintError".
Sadly, this doesn't work for Perl versions up to and including
5.004_04. Even more sadly, for Perl 5.5 and 5.6.0 it does work but
leaks memory! For backwards compatibility, you could just use "eval
{ ... }" instead.
"HandleError" (code ref, inherited)
The "HandleError" attribute can be used to provide your own
alternative behaviour in case of errors. If set to a reference to a
subroutine then that subroutine is called when an error is detected
(at the same point that "RaiseError" and "PrintError" are handled).
The subroutine is called with three parameters: the error message
string that "RaiseError" and "PrintError" would use, the DBI handle
being used, and the first value being returned by the method that
failed (typically undef).
If the subroutine returns a false value then the "RaiseError" and/or
"PrintError" attributes are checked and acted upon as normal.
For example, to "die" with a full stack trace for any error:
use Carp;
$h->{HandleError} = sub { confess(shift) };
Or to turn errors into exceptions:
use Exception; # or your own favourite exception module
$h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };
It is possible to 'stack' multiple HandleError handlers by using
closures:
sub your_subroutine {
my $previous_handler = $h->{HandleError};
$h->{HandleError} = sub {
return 1 if $previous_handler and &$previous_handler(@_);
... your code here ...
};
}
Using a "my" inside a subroutine to store the previous "HandleError"
value is important. See perlsub and perlref for more information
about *closures*.
It is possible for "HandleError" to alter the error message that
will be used by "RaiseError" and "PrintError" if it returns false.
It can do that by altering the value of $_[0]. This example appends
a stack trace to all errors and, unlike the previous example using
Carp::confess, this will work "PrintError" as well as "RaiseError":
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for "HandleError" to hide an error, to a limited
degree, by using "set_err" to reset $DBI::err and $DBI::errstr, and
altering the return value of the failed method. For example:
$h->{HandleError} = sub {
return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
return 0 unless $_[1]->err == 1234; # the error to 'hide'
$h->set_err(0,""); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single value and is hard
to make reliable (avoiding infinite loops, for example) and so isn't
recommended for general use! If you find a *good* use for it then
please let me know.
"ShowErrorStatement" (boolean, inherited)
The "ShowErrorStatement" attribute can be used to cause the relevant
Statement text to be appended to the error messages generated by the
"RaiseError" and "PrintError" attributes. Only applies to errors on
statement handles plus the prepare(), do(), and the various
"select*()" database handle methods. (The exact format of the
appended text is subject to change.)
If "$h->{ParamValues}" returns a hash reference of parameter
(placeholder) values then those are formatted and appended to the
end of the Statement text in the error message.
"TraceLevel" (integer, inherited)
The "TraceLevel" attribute can be used as an alternative to the
"trace" method to set the DBI trace level for a specific handle.
"FetchHashKeyName" (string, inherited)
The "FetchHashKeyName" attribute is used to specify whether the
fetchrow_hashref() method should perform case conversion on the
field names used for the hash keys. For historical reasons it
defaults to '"NAME"' but it is recommended to set it to '"NAME_lc"'
(convert to lower case) or '"NAME_uc"' (convert to upper case)
according to your preference. It can only be set for driver and
database handles. For statement handles the value is frozen when
prepare() is called.
"ChopBlanks" (boolean, inherited)
The "ChopBlanks" attribute can be used to control the trimming of
trailing space characters from fixed width character (CHAR) fields.
No other field types are affected, even where field values have
trailing spaces.
The default is false (although it is possible that the default may
change). Applications that need specific behaviour should set the
attribute as needed. Emulation interfaces should set the attribute
to match the behaviour of the interface they are emulating.
Drivers are not required to support this attribute, but any driver
which does not support it must arrange to return "undef" as the
attribute value.
"LongReadLen" (unsigned integer, inherited)
The "LongReadLen" attribute may be used to control the maximum
length of long fields ("blob", "memo", etc.) which the driver will
read from the database automatically when it fetches each row of
data. The "LongReadLen" attribute only relates to fetching and
reading long values; it is not involved in inserting or updating
them.
A value of 0 means not to automatically fetch any long data.
("fetch" should return "undef" for long fields when "LongReadLen" is
0.)
The default is typically 0 (zero) bytes but may vary between
drivers. Applications fetching long fields should set this value to
slightly larger than the longest long field value to be fetched.
Some databases return some long types encoded as pairs of hex
digits. For these types, "LongReadLen" relates to the underlying
data length and not the doubled-up length of the encoded string.
Changing the value of "LongReadLen" for a statement handle after it
has been "prepare"'d will typically have no effect, so it's common
to set "LongReadLen" on the $dbh before calling "prepare".
Note that the value used here has a direct effect on the memory used
by the application, so don't be too generous.
See "LongTruncOk" for more information on truncation behaviour.
"LongTruncOk" (boolean, inherited)
The "LongTruncOk" attribute may be used to control the effect of
fetching a long field value which has been truncated (typically
because it's longer than the value of the "LongReadLen" attribute).
By default, "LongTruncOk" is false and so fetching a long value that
needs to be truncated will cause the fetch to fail. (Applications
should always be sure to check for errors after a fetch loop in case
an error, such as a divide by zero or long field truncation, caused
the fetch to terminate prematurely.)
If a fetch fails due to a long field truncation when "LongTruncOk"
is false, many drivers will allow you to continue fetching further
rows.
See also "LongReadLen".
"TaintIn" (boolean, inherited)
If the "TaintIn" attribute is set to a true value *and* Perl is
running in taint mode (e.g., started with the "-T" option), then all
the arguments to most DBI method calls are checked for being
tainted. *This may change.*
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintIn
attribute, for that statement handle, for the duration of the fetch
loop.
The "TaintIn" attribute was added in DBI 1.31.
"TaintOut" (boolean, inherited)
If the "TaintOut" attribute is set to a true value *and* Perl is
running in taint mode (e.g., started with the "-T" option), then
most data fetched from the database is considered tainted. *This may
change.*
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintOut
attribute, for that statement handle, for the duration of the fetch
loop.
Currently only fetched data is tainted. It is possible that the
results of other DBI method calls, and the value of fetched
attributes, may also be tainted in future versions. That change may
well break your applications unless you take great care now. If you
use DBI Taint mode, please report your experience and any
suggestions for changes.
The "TaintOut" attribute was added in DBI 1.31.
"Taint" (boolean, inherited)
The "Taint" attribute is a shortcut for "TaintIn" and "TaintOut" (it
is also present for backwards compatibility).
Setting this attribute sets both "TaintIn" and "TaintOut", and
retrieving it returns a true value if and only if "TaintIn" and
"TaintOut" are both set to true values.
"Profile" (inherited)
The "Profile" attribute enables the collection and reporting of
method call timing statistics. See the DBI::Profile module
documentation for *much* more detail.
The "Profile" attribute was added in DBI 1.24.
"private_your_module_name_*"
The DBI provides a way to store extra information in a DBI handle as
"private" attributes. The DBI will allow you to store and retrieve
any attribute which has a name starting with ""private_"".
It is *strongly* recommended that you use just *one* private
attribute (e.g., use a hash ref) *and* give it a long and
unambiguous name that includes the module or application name that
the attribute relates to (e.g.,
""private_YourFullModuleName_thingy"").
Because of the way the Perl tie mechanism works you cannot reliably
use the "||=" operator directly to initialise the attribute, like
this:
my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG
you should use a two step approach like this:
my $foo = $dbh->{private_yourmodname_foo};
$foo ||= $dbh->{private_yourmodname_foo} = { ... };
This attribute is primarily of interest to people sub-classing DBI.
DBI DATABASE HANDLE OBJECTS
This section covers the methods and attributes associated with database
handles.
Database Handle Methods
The following methods are specified for DBI database handles:
"clone"
$new_dbh = $dbh->clone();
$new_dbh = $dbh->clone(\%attr);
The "clone" method duplicates the $dbh connection by connecting with
the same parameters ($dsn, $user, $password) as originally used.
The attributes for the cloned connect are the same as those used for
the original connect, with some other attribute merged over them
depending on the \%attr parameter.
If \%attr is given then the attributes it contains are merged into
the original attributes and override any with the same names.
Effectively the same as doing:
%attribues_used = ( %original_attributes, %attr );
If \%attr is not given then it defaults to a hash containing all the
attributes in the attribute cache of $dbh excluding any non-code
references, plus the main boolean attributes (RaiseError,
PrintError, AutoCommit, etc.). This behaviour is subject to change.
The clone method can be used even if the database handle is
disconnected.
The "clone" method was added in DBI 1.33. It is very new and likely
to change.
"do"
$rows = $dbh->do($statement) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr, @bind_values) or die ...
Prepare and execute a single statement. Returns the number of rows
affected or "undef" on error. A return value of -1 means the number
of rows is not known, not applicable, or not available.
This method is typically most useful for *non*-"SELECT" statements
that either cannot be prepared in advance (due to a limitation of
the driver) or do not need to be executed repeatedly. It should not
be used for "SELECT" statements because it does not return a
statement handle (so you can't fetch any data).
The default "do" method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement, $attr) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows; # always return true if no error
}
For example:
my $rows_deleted = $dbh->do(q{
DELETE FROM table
WHERE status = ?
}, undef, 'DONE') or die $dbh->errstr;
Using placeholders and @bind_values with the "do" method can be
useful because it avoids the need to correctly quote any variables
in the $statement. But if you'll be executing the statement many
times then it's more efficient to "prepare" it once and call
"execute" many times instead.
The "q{...}" style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the
double-quote-like "qq{...}" operator if you want to interpolate
variables into the string. See "Quote and Quote-like Operators" in
perlop for more details.
"selectrow_array"
@row_ary = $dbh->selectrow_array($statement);
@row_ary = $dbh->selectrow_array($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_array" into a single call. If called in a list context, it
returns the first row of data from the statement. The $statement
parameter can be a previously prepared statement handle, in which
case the "prepare" is skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return an empty list.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in a
scalar context, an "undef" is returned if there are no more rows or
if an error occurred. That "undef" can't be distinguished from an
"undef" returned because the first field value was NULL. For these
reasons you should exercise some caution if you use
"selectrow_array" in a scalar context.
"selectrow_arrayref"
$ary_ref = $dbh->selectrow_arrayref($statement);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_arrayref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a
previously prepared statement handle, in which case the "prepare" is
skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return undef.
"selectrow_hashref"
$hash_ref = $dbh->selectrow_hashref($statement);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_hashref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a
previously prepared statement handle, in which case the "prepare" is
skipped.
If any method fails, and "RaiseError" is not set,
"selectrow_hashref" will return undef.
"selectall_arrayref"
$ary_ref = $dbh->selectall_arrayref($statement);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchall_arrayref" into a single call. It returns a reference to an
array containing a reference to an array for each row of data
fetched.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If "RaiseError" is not set and any method except "fetchall_arrayref"
fails then "selectall_arrayref" will return "undef"; if
"fetchall_arrayref" fails then it will return with whatever data has
been fetched thus far. You should check "$sth->err" afterwards (or
use the "RaiseError" attribute) to discover if the data is complete
or was truncated due to an error.
The "fetchall_arrayref" method called by "selectall_arrayref"
supports a $max_rows parameter. You can specify a value for
$max_rows by including a '"MaxRows"' attribute in \%attr.
The "fetchall_arrayref" method called by "selectall_arrayref" also
supports a $slice parameter. You can specify a value for $slice by
including a '"Slice"' or '"Columns"' attribute in \%attr. The only
difference between the two is that if "Slice" is not defined and
"Columns" is an array ref, then the array is assumed to contain
column index values (which count from 1), rather than perl array
index values. In which case the array is copied and each value
decremented before passing to "/fetchall_arrayref".
"selectall_hashref"
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchall_hashref" into a single call. It returns a reference to a
hash containing one entry for each row. The key for each row entry
is specified by $key_field. The value is a reference to a hash
returned by "fetchrow_hashref".
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetchrow_hashref" fails, and "RaiseError" is
not set, "selectall_hashref" will return "undef". If
"fetchrow_hashref" fails and "RaiseError" is not set, then it will
return with whatever data it has fetched thus far. $DBI::err should
be checked to catch that.
"selectcol_arrayref"
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute", and fetching one
column from all the rows, into a single call. It returns a reference
to an array containing the values of the first column from each row.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetch" fails, and "RaiseError" is not set,
"selectcol_arrayref" will return "undef". If "fetch" fails and
"RaiseError" is not set, then it will return with whatever data it
has fetched thus far. $DBI::err should be checked to catch that.
The "selectcol_arrayref" method defaults to pushing a single column
value (the first) from each row into the result array. However, it
can also push another column, or even multiple columns per row, into
the result array. This behaviour can be specified via a '"Columns"'
attribute which must be a ref to an array containing the column
number or numbers to use. For example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name
"prepare"
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a statement for later execution by the database engine and
returns a reference to a statement handle object.
The returned statement handle can be used to get attributes of the
statement and invoke the "execute" method. See "Statement Handle
Methods".
Drivers for engines without the concept of preparing a statement
will typically just store the statement in the returned handle and
process it when "$sth->execute" is called. Such drivers are unlikely
to give much useful information about the statement, such as
"$sth->{NUM_OF_FIELDS}", until after "$sth->execute" has been
called. Portable applications should take this into account.
In general, DBI drivers do not parse the contents of the statement
(other than simply counting any "Placeholders"). The statement is
passed directly to the database engine, sometimes known as pass-thru
mode. This has advantages and disadvantages. On the plus side, you
can access all the functionality of the engine being used. On the
downside, you're limited if you're using a simple engine, and you
need to take extra care if writing applications intended to be
portable between engines.
Portable applications should not assume that a new statement can be
prepared and/or executed while still fetching results from a
previous statement.
Some command-line SQL tools use statement terminators, like a
semicolon, to indicate the end of a statement. Such terminators
should not normally be used with the DBI.
"prepare_cached"
$sth = $dbh->prepare_cached($statement)
$sth = $dbh->prepare_cached($statement, \%attr)
$sth = $dbh->prepare_cached($statement, \%attr, $allow_active)
Like "prepare" except that the statement handle returned will be
stored in a hash associated with the $dbh. If another call is made
to "prepare_cached" with the same $statement and %attr values, then
the corresponding cached $sth will be returned without contacting
the database server.
Here are some examples of "prepare_cached":
sub insert_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $sql = sprintf "insert into %s (%s) values (%s)",
$table, join(",", @fields), join(",", ("?")x@fields);
my $sth = $dbh->prepare_cached($sql);
return $sth->execute(@values);
}
sub search_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $qualifier = "";
$qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
$sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
return $dbh->selectall_arrayref($sth, {}, @values);
}
*Caveat emptor:* This caching can be useful in some applications,
but it can also cause problems and should be used with care. Here is
a contrived case where caching would cause a significant problem:
my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth->execute($bar);
while (my $data = $sth->fetchrow_hashref) {
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute($data->{bar});
while (my $data2 = $sth2->fetchrow_arrayref) {
do_stuff(...);
}
}
In this example, since both handles are preparing the exact same
statement, $sth2 will not be its own statement handle, but a
duplicate of $sth returned from the cache. The results will
certainly not be what you expect. Typically the the inner fetch loop
will work normally, fetching all the records and terminating when
there are no more, but now $sth is the same as $sth2 the outer fetch
loop will also terminate.
The $allow_active parameter lets you adjust DBI's behaviour when
prepare_cached is returning a statement handle that is still active.
There are three settings:
0: A warning will be generated, and "finish" will be called on
the statement handle before it is returned. This is the default
behaviour if $allow_active is not passed.
1: "finish" will be called on the statement handle, but the
warning is suppressed.
2: DBI will not touch the statement handle before returning it.
You will need to check "$sth->{Active}" on the returned
statement handle and deal with it in your own code.
Because the cache used by prepare_cached() is keyed by all the
parameters, including any attributes passed, you can also avoid this
issue by doing something like:
my $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });
which will ensure that prepare_cached only returns statements cached
by that line of code in that source file.
"commit"
$rc = $dbh->commit or die $dbh->errstr;
Commit (make permanent) the most recent series of database changes
if the database supports transactions and AutoCommit is off.
If "AutoCommit" is on, then calling "commit" will issue a "commit
ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"rollback"
$rc = $dbh->rollback or die $dbh->errstr;
Rollback (undo) the most recent series of uncommitted database
changes if the database supports transactions and AutoCommit is off.
If "AutoCommit" is on, then calling "rollback" will issue a
"rollback ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"begin_work"
$rc = $dbh->begin_work or die $dbh->errstr;
Enable transactions (by turning "AutoCommit" off) until the next
call to "commit" or "rollback". After the next "commit" or
"rollback", "AutoCommit" will automatically be turned on again.
If "AutoCommit" is already off when "begin_work" is called then it
does nothing except return an error. If the driver does not support
transactions then when "begin_work" attempts to set "AutoCommit" off
the driver will trigger a fatal error.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"disconnect"
$rc = $dbh->disconnect or warn $dbh->errstr;
Disconnects the database from the database handle. "disconnect" is
typically only used before exiting the program. The handle is of
little use after disconnecting.
The transaction behaviour of the "disconnect" method is, sadly,
undefined. Some database systems (such as Oracle and Ingres) will
automatically commit any outstanding changes, but others (such as
Informix) will rollback any outstanding changes. Applications not
using "AutoCommit" should explicitly call "commit" or "rollback"
before calling "disconnect".
The database is automatically disconnected by the "DESTROY" method
if still connected when there are no longer any references to the
handle. The "DESTROY" method for each driver should implicitly call
"rollback" to undo any uncommitted changes. This is vital behaviour
to ensure that incomplete transactions don't get committed simply
because Perl calls "DESTROY" on every object before exiting. Also,
do not rely on the order of object destruction during "global
destruction", as it is undefined.
Generally, if you want your changes to be commited or rolled back
when you disconnect, then you should explicitly call "commit" or
"rollback" before disconnecting.
If you disconnect from a database while you still have active
statement handles (e.g., SELECT statement handles that may have more
data to fetch), you will get a warning. The warning may indicate
that a fetch loop terminated early, perhaps due to an uncaught
error. To avoid the warning call the "finish" method on the active
handles.
"ping"
$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way, if the
database server is still running and the connection to it is still
working. Individual drivers should implement this function in the
most suitable manner for their database engine.
The current *default* implementation always returns true without
actually doing anything. Actually, it returns ""0 but true"" which
is true but zero. That way you can tell if the return value is
genuine or just the default. Drivers should override this method
with one that does the right thing for their type of database.
Few applications would have direct use for this method. See the
specialized Apache::DBI module for one example usage.
"get_info"
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e. driver and data
source capabilities, restrictions etc. It returns "undef" for
unknown or unimplemented information types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER
$max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT
See "Standards Reference Information" for more detailed information
about the information types and their meanings and possible return
values.
The DBI curently doesn't provide a name to number mapping for the
information type codes or the results. Applications are expected to
use the integer values directly, with the name in a comment, or
define their own named values using something like the constant
pragma.
Because some DBI methods make use of get_info(), drivers are
strongly encouraged to support *at least* the following very minimal
set of information types to ensure the DBI itself works properly:
Type Name Example A Example B
---- -------------------------- ------------ ------------
17 SQL_DBMS_NAME 'ACCESS' 'Oracle'
18 SQL_DBMS_VER '03.50.0000' '08.01.0721'
29 SQL_IDENTIFIER_QUOTE_CHAR '`' '"'
41 SQL_CATALOG_NAME_SEPARATOR '.' '@'
114 SQL_CATALOG_LOCATION 1 2
"table_info"
Warning: This method is experimental and may change.
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
$sth = $dbh->table_info( \%attr ); # old style
Returns an active statement handle that can be used to fetch
information about tables and views that exist in the database.
The old style interface passes all the parameters as a reference to
an attribute hash with some or all of the following attributes:
%attr = (
TABLE_CAT => $catalog # String value of the catalog name
, TABLE_SCHEM => $schema # String value of the schema name
, TABLE_NAME => $table # String value of the table name
, TABLE_TYPE => $type # String value of the table type(s)
);
The old style interface is deprecated and will be removed in a
future version.
The support for the selection criteria is driver specific. If the
driver doesn't support one or more of them then you may get back
more than you asked for and can do the filtering yourself.
The arguments $catalog, $schema and $table may accept search
patterns according to the database/driver, for example: $table =
'%FOO%'; Remember that the underscore character ('"_"') is a search
pattern that means match any character, so 'FOO_%' is the same as
'FOO%' and 'FOO_BAR%' will match names like 'FOO1BAR'.
The value of $type is a comma-separated list of one or more types of
tables to be returned in the result set. Each value may optionally
be quoted, e.g.:
$type = "TABLE";
$type = "'TABLE','VIEW'";
In addition the following special cases may also be supported by
some drivers:
* If the value of $catalog is '%' and $schema and $table name are
empty strings, the result set contains a list of catalog names. For
example:
$sth = $dbh->table_info('%', '', '');
* If the value of $schema is '%' and $catalog and $table are empty
strings, the result set contains a list of schema names.
* If the value of $type is '%' and $catalog, $schema, and $table are
all empty strings, the result set contains a list of table types.
The statement handle returned has at least the following fields in
the order show below. Other fields, after these, may also be
present.
TABLE_CAT: Table catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is usually the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The name of the schema containing the TABLE_NAME value.
This field is NULL ("undef") if not applicable to data source, and
empty if not applicable to the table.
TABLE_NAME: Name of the table (or view, synonym, etc).
TABLE_TYPE: One of the following: "TABLE", "VIEW", "SYSTEM TABLE",
"GLOBAL TEMPORARY", "LOCAL TEMPORARY", "ALIAS", "SYNONYM" or a type
identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL ("undef").
Note that "table_info" might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by "table_info".
See also "tables", "Catalog Methods" and "Standards Reference
Information".
"column_info"
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to fetch
information about columns in specified tables.
The arguments $schema, $table and $column may accept search patterns
according to the database/driver, for example: $table = '%FOO%';
Note: The support for the selection criteria is driver specific. If
the driver doesn't support one or more of them then you may get back
more than you asked for and can do the filtering yourself.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef") if
not applicable to the data source, and empty if not applicable to
the table.
TABLE_NAME: The table identifier. Note: A driver may provide column
metadata not only for base tables, but also for derived objects like
SYNONYMS etc.
COLUMN_NAME: The column identifier.
DATA_TYPE: The concise data type code.
TYPE_NAME: A data source dependent data type name.
COLUMN_SIZE: The column size. This is the maximum length in
characters for character data types, the number of digits or bits
for numeric data types or the length in the representation of
temporal types. See the relevant specifications for detailed
information.
BUFFER_LENGTH: The length in bytes of transferred data.
DECIMAL_DIGITS: The total number of significant digits to the right
of the decimal point.
NUM_PREC_RADIX: The radix for numeric precision. The value is 10 or
2 for numeric data types and NULL ("undef") if not applicable.
NULLABLE: Indicates if a column can accept NULLs. The following
values are defined:
SQL_NO_NULLS 0
SQL_NULLABLE 1
SQL_NULLABLE_UNKNOWN 2
REMARKS: A description of the column.
COLUMN_DEF: The default value of the column.
SQL_DATA_TYPE: The SQL data type.
SQL_DATETIME_SUB: The subtype code for datetime and interval data
types.
CHAR_OCTET_LENGTH: The maximum length in bytes of a character or
binary data type column.
ORDINAL_POSITION: The column sequence number (starting with 1).
IS_NULLABLE: Indicates if the column can accept NULLs. Possible
values are: 'NO', 'YES' and ''.
SQL/CLI defines the following additional columns:
CHAR_SET_CAT
CHAR_SET_SCHEM
CHAR_SET_NAME
COLLATION_CAT
COLLATION_SCHEM
COLLATION_NAME
UDT_CAT
UDT_SCHEM
UDT_NAME
DOMAIN_CAT
DOMAIN_SCHEM
DOMAIN_NAME
SCOPE_CAT
SCOPE_SCHEM
SCOPE_NAME
MAX_CARDINALITY
DTD_IDENTIFIER
IS_SELF_REF
Drivers capable of supplying any of those values should do so in the
corresponding column and supply undef values for the others.
Drivers wishing to provide extra database/driver specific
information should do so in extra columns beyond all those listed
above, and use lowercase field names with the driver-specific prefix
(i.e., 'ora_...'). Applications accessing such fields should do so
by name and not by column number.
The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and
ORDINAL_POSITION.
Note: There is some overlap with statement attributes (in perl) and
SQLDescribeCol (in ODBC). However, SQLColumns provides more
metadata.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key_info"
Warning: This method is experimental and may change.
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to fetch
information about columns that make up the primary key for a table.
The arguments don't accept search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' );
$data = $sth->fetchall_arrayref;
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef") if
not applicable to the data source, and empty if not applicable to
the table.
TABLE_NAME: The table identifier.
COLUMN_NAME: The column identifier.
KEY_SEQ: The column sequence number (starting with 1). Note: This
field is named ORDINAL_POSITION in SQL/CLI.
PK_NAME: The primary key constraint identifier. This field is NULL
("undef") if not applicable to the data source.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key"
@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method. Returns a list of
the column names that comprise the primary key of the specified
table. The list is in primary key column sequence order.
"foreign_key_info"
Warning: This method is experimental and may change.
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
Returns an active statement handle that can be used to fetch
information about foreign keys in and/or referencing the specified
table(s). The arguments don't accept search patterns (unlike
table_info()).
$pk_catalog, $pk_schema, $pk_table identify the primary (unique) key
table (PKT).
$fk_catalog, $fk_schema, $fk_table identify the foreign key table
(FKT).
If both PKT and FKT are given, the function returns the foreign key,
if any, in table FKT that refers to the primary (unique) key of
table PKT. (Note: In SQL/CLI, the result is implementation-defined.)
If only PKT is given, then the result set contains the primary key
of that table and all foreign keys that refer to it.
If only FKT is given, then the result set contains all foreign keys
in that table and the primary keys to which they refer. (Note: In
SQL/CLI, the result includes unique keys too.)
For example:
$sth = $dbh->foreign_key_info( undef, $user, 'master');
$sth = $dbh->foreign_key_info( undef, undef, undef , undef, $user, 'detail');
$sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has the following fields in the order
shown below. Because ODBC never includes unique keys, they define
different columns in the result set than SQL/CLI. SQL/CLI column
names are shown in parentheses.
PKTABLE_CAT ( UK_TABLE_CAT ): The primary (unique) key table catalog
identifier. This field is NULL ("undef") if not applicable to the
data source, which is often the case. This field is empty if not
applicable to the table.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary (unique) key table
schema identifier. This field is NULL ("undef") if not applicable to
the data source, and empty if not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key table
identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key column
identifier.
FKTABLE_CAT ( FK_TABLE_CAT ): The foreign key table catalog
identifier. This field is NULL ("undef") if not applicable to the
data source, which is often the case. This field is empty if not
applicable to the table.
FKTABLE_SCHEM ( FK_TABLE_SCHEM ): The foreign key table schema
identifier. This field is NULL ("undef") if not applicable to the
data source, and empty if not applicable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identifier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column identifier.
KEY_SEQ ( ORDINAL_POSITION ): The column sequence number (starting
with 1).
UPDATE_RULE ( UPDATE_RULE ): The referential action for the UPDATE
rule. The following codes are defined:
CASCADE 0
RESTRICT 1
SET NULL 2
NO ACTION 3
SET DEFAULT 4
DELETE_RULE ( DELETE_RULE ): The referential action for the DELETE
rule. The codes are the same as for UPDATE_RULE.
FK_NAME ( FK_NAME ): The foreign key name.
PK_NAME ( UK_NAME ): The primary (unique) key name.
DEFERRABILITY ( DEFERABILITY ): The deferrability of the foreign key
constraint. The following codes are defined:
INITIALLY DEFERRED 5
INITIALLY IMMEDIATE 6
NOT DEFERRABLE 7
( UNIQUE_OR_PRIMARY ): This column is necessary if a driver
includes all candidate (i.e. primary and alternate) keys in the
result set (as specified by SQL/CLI). The value of this column is
UNIQUE if the foreign key references an alternate key and PRIMARY if
the foreign key references a primary key, or it may be undefined if
the driver doesn't have access to the information.
See also "Catalog Methods" and "Standards Reference Information".
"tables"
@names = $dbh->tables( $catalog, $schema, $table, $type );
@names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of matching table
names, possibly including a catalog/schema prefix.
See "table_info" for a description of the parameters.
If "$dbh->get_info(29)" returns true (29 is
SQL_IDENTIFIER_QUOTE_CHAR) then the table names are constructed and
quoted by "quote_identifier" to ensure they are usable even if they
contain whitespace or reserved words etc.
"type_info_all"
$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds information about each
data type variant supported by the database and driver. The array
and its contents should be treated as read-only.
The first item is a reference to an 'index' hash of "Name =">
"Index" pairs. The items following that are references to arrays,
one per supported data type variant. The leading index hash defines
the names and order of the fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
SQL_DATA_TYPE => 16,
SQL_DATETIME_SUB => 17,
NUM_PREC_RADIX => 18,
INTERVAL_PRECISION=> 19,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in the
"DATA_TYPE" field if there are different ways to spell the type name
and/or there are variants of the type with different attributes
(e.g., with and without "AUTO_UNIQUE_VALUE" set, with and without
"UNSIGNED_ATTRIBUTE", etc).
The rows are ordered by "DATA_TYPE" first and then by how closely
each type maps to the corresponding ODBC SQL data type, closest
first.
The meaning of the fields is described in the documentation for the
"type_info" method. The index values shown above (e.g., "NULLABLE
="> 6) are for illustration only. Drivers may define the fields with
a different order.
This method is not normally used directly. The "type_info" method
provides a more useful interface to the data.
Even though an 'index' hash is provided, all the field names in the
index hash defined above will always have the index values defined
above. This is defined behaviour so that you don't need to rely on
the index hash, which is handy because the lettercase of the keys is
not defined. It is usually uppercase, as show here, but drivers are
free to return names with any lettercase. Drivers are also free to
return extra driver-specific columns of information - though it's
recommended that they start at column index 50 to leave room for
expansion of the DBI/ODBC specification.
"type_info"
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or
more variants of $data_type. The list is ordered by "DATA_TYPE"
first and then by how closely each type maps to the corresponding
ODBC SQL data type, closest first. If called in a scalar context
then only the first (best) element is returned.
If $data_type is undefined or "SQL_ALL_TYPES", then the list will
contain hashes for all data type variants supported by the database
and driver.
If $data_type is an array reference then "type_info" returns the
information for the *first* type in the array that has any matches.
The keys of the hash follow the same letter case conventions as the
rest of the DBI (see "Naming Conventions and Name Space"). The
following items should exist:
TYPE_NAME (string)
Data type name for use in CREATE TABLE statements etc.
DATA_TYPE (integer)
SQL data type number.
COLUMN_SIZE (integer)
For numeric types, this is either the total number of digits (if
the NUM_PREC_RADIX value is 10) or the total number of bits
allowed in the column (if NUM_PREC_RADIX is 2).
For string types, this is the maximum size of the string in
bytes.
For date and interval types, this is the maximum number of
characters needed to display the value.
LITERAL_PREFIX (string)
Characters used to prefix a literal. A typical prefix is ""'""
for characters, or possibly ""0x"" for binary values passed as
hexadecimal. NULL ("undef") is returned for data types for which
this is not applicable.
LITERAL_SUFFIX (string)
Characters used to suffix a literal. Typically ""'"" for
characters. NULL ("undef") is returned for data types where this
is not applicable.
CREATE_PARAMS (string)
Parameter names for data type definition. For example,
"CREATE_PARAMS" for a "DECIMAL" would be ""precision,scale"" if
the DECIMAL type should be declared as
"DECIMAL("*precision,scale*")" where *precision* and *scale* are
integer values. For a "VARCHAR" it would be ""max length"". NULL
("undef") is returned for data types for which this is not
applicable.
NULLABLE (integer)
Indicates whether the data type accepts a NULL value: 0 or an
empty string = no, 1 = yes, 2 = unknown.
CASE_SENSITIVE (boolean)
Indicates whether the data type is case sensitive in collations
and comparisons.
SEARCHABLE (integer)
Indicates how the data type can be used in a WHERE clause, as
follows:
0 - Cannot be used in a WHERE clause
1 - Only with a LIKE predicate
2 - All comparison operators except LIKE
3 - Can be used in a WHERE clause with any comparison operator
UNSIGNED_ATTRIBUTE (boolean)
Indicates whether the data type is unsigned. NULL ("undef") is
returned for data types for which this is not applicable.
FIXED_PREC_SCALE (boolean)
Indicates whether the data type always has the same precision
and scale (such as a money type). NULL ("undef") is returned for
data types for which this is not applicable.
AUTO_UNIQUE_VALUE (boolean)
Indicates whether a column of this data type is automatically
set to a unique value whenever a new row is inserted. NULL
("undef") is returned for data types for which this is not
applicable.
LOCAL_TYPE_NAME (string)
Localized version of the "TYPE_NAME" for use in dialog with
users. NULL ("undef") is returned if a localized name is not
available (in which case "TYPE_NAME" should be used).
MINIMUM_SCALE (integer)
The minimum scale of the data type. If a data type has a fixed
scale, then "MAXIMUM_SCALE" holds the same value. NULL ("undef")
is returned for data types for which this is not applicable.
MAXIMUM_SCALE (integer)
The maximum scale of the data type. If a data type has a fixed
scale, then "MINIMUM_SCALE" holds the same value. NULL ("undef")
is returned for data types for which this is not applicable.
SQL_DATA_TYPE (integer)
This column is the same as the "DATA_TYPE" column, except for
interval and datetime data types. For interval and datetime data
types, the "SQL_DATA_TYPE" field will return "SQL_INTERVAL" or
"SQL_DATETIME", and the "SQL_DATETIME_SUB" field below will
return the subcode for the specific interval or datetime data
type. If this field is NULL, then the driver does not support or
report on interval or datetime subtypes.
SQL_DATETIME_SUB (integer)
For interval or datetime data types, where the "SQL_DATA_TYPE"
field above is "SQL_INTERVAL" or "SQL_DATETIME", this field will
hold the *subcode* for the specific interval or datetime data
type. Otherwise it will be NULL ("undef").
Although not mentioned explicitly in the standards, it seems
there is a simple relationship between these values:
DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate numeric types,
"NUM_PREC_RADIX" contains the value 2 and "COLUMN_SIZE" holds
the number of bits. For exact numeric types, "NUM_PREC_RADIX"
contains the value 10 and "COLUMN_SIZE" holds the number of
decimal digits. NULL ("undef") is returned either for data types
for which this is not applicable or if the driver cannot report
this information.
INTERVAL_PRECISION (integer)
The interval leading precision for interval types. NULL is
returned either for data types for which this is not applicable
or if the driver cannot report this information.
For example, to find the type name for the fields in a select
statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }
Since DBI and ODBC drivers vary in how they map their types into the
ISO standard types you may need to search for more than one type.
Here's an example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small integers, you
could use a list starting with "SQL_SMALLINT", "SQL_INTEGER",
"SQL_DECIMAL", etc.
See also "Standards Reference Information".
"quote"
$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in an SQL
statement, by escaping any special characters (such as quotation
marks) contained within the string and adding the required type of
outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t' (including the
outer quotation marks).
An undefined $value value will be returned as the string "NULL"
(without single quotation marks) to match how NULLs are represented
in SQL.
If $data_type is supplied, it is used to try to determine the
required quoting behaviour by using the information returned by
"type_info". As a special case, the standard numeric types are
optimized to return $value without calling "type_info".
Quote will probably *not* be able to deal with all possible input
(such as binary data or data containing newlines), and is not
related in any way with escaping or quoting shell meta-characters.
There is no need to quote values being used with "Placeholders and
Bind Values".
"quote_identifier"
$sql = $dbh->quote_identifier( $name );
$sql = $dbh->quote_identifier( $catalog, $schema, $table, \%attr );
Quote an identifier (table name etc.) for use in an SQL statement,
by escaping any special characters (such as double quotation marks)
it contains and adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are quoted and then
joined together, typically with a dot (".") character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her schema"."My table"
(including all the double quotation marks).
If three names are supplied then the first is assumed to be a
catalog name and special rules may be applied based on what
"get_info" returns for SQL_CATALOG_NAME_SEPARATOR (41) and
SQL_CATALOG_LOCATION (114). For example, for Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
Database Handle Attributes
This section describes attributes specific to database handles.
Changes to these database handle attributes do not affect any other
existing or future database handles.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver-specific attributes (which all have names
starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
"AutoCommit" (boolean)
If true, then database changes cannot be rolled-back (undone). If
false, then database changes automatically occur within a
"transaction", which must either be committed or rolled back using
the "commit" or "rollback" methods.
Drivers should always default to "AutoCommit" mode (an unfortunate
choice largely forced on the DBI by ODBC and JDBC conventions.)
Attempting to set "AutoCommit" to an unsupported value is a fatal
error. This is an important feature of the DBI. Applications that
need full transaction behaviour can set "$dbh->{AutoCommit} = 0" (or
set "AutoCommit" to 0 via "connect") without having to check that
the value was assigned successfully.
For the purposes of this description, we can divide databases into
three categories:
Databases which don't support transactions at all.
Databases in which a transaction is always active.
Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn "AutoCommit" off is a fatal
error. "commit" and "rollback" both issue warnings about being
ineffective while "AutoCommit" is in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational databases with
"ANSI standard" transaction behaviour. If "AutoCommit" is off, then
changes to the database won't have any lasting effect unless
"commit" is called (but see also "disconnect"). If "rollback" is
called then any changes since the last commit are undone.
If "AutoCommit" is on, then the effect is the same as if the DBI
called "commit" automatically after every successful database
operation. So calling "commit" or "rollback" explicitly while
"AutoCommit" is on would be ineffective because the changes would
have already been commited.
Changing "AutoCommit" from off to on will trigger a "commit".
For databases which don't support a specific auto-commit mode, the
driver has to commit each statement automatically using an explicit
"COMMIT" after it completes successfully (and roll it back using an
explicit "ROLLBACK" if it fails). The error information reported to
the application will correspond to the statement which was executed,
unless it succeeded and the commit or rollback failed.
* Databases in which a transaction must be explicitly started
For these databases, the intention is to have them act like
databases in which a transaction is always active (as described
above).
To do this, the driver will automatically begin an explicit
transaction when "AutoCommit" is turned off, or after a "commit" or
"rollback" (or when the application issues the next database
operation after one of those events).
In this way, the application does not have to treat these databases
as a special case.
See "commit", "disconnect" and "Transactions" for other important
notes about transactions.
"Driver" (handle)
Holds the handle of the parent driver. The only recommended use for
this is to find the name of the driver using:
$dbh->{Driver}->{Name}
"Name" (string)
Holds the "name" of the database. Usually (and recommended to be)
the same as the ""dbi:DriverName:..."" string used to connect to the
database, but with the leading ""dbi:DriverName:"" removed.
"Statement" (string, read-only)
Returns the statement string passed to the most recent "prepare"
method called in this database handle, even if that method failed.
This is especially useful where "RaiseError" is enabled and the
exception handler checks $@ and sees that a 'prepare' method call
failed.
"RowCacheSize" (integer)
A hint to the driver indicating the size of the local row cache that
the application would like the driver to use for future "SELECT"
statements. If a row cache is not implemented, then setting
"RowCacheSize" is ignored and getting the value returns "undef".
Some "RowCacheSize" values have special meaning, as follows:
0 - Automatically determine a reasonable cache size for each C<SELECT>
1 - Disable the local row cache
>1 - Cache this many rows
<0 - Cache as many rows that will fit into this much memory for each C<SELECT>.
Note that large cache sizes may require a very large amount of
memory (*cached rows * maximum size of row*). Also, a large cache
will cause a longer delay not only for the first fetch, but also
whenever the cache needs refilling.
See also the "RowsInCache" statement handle attribute.
DBI STATEMENT HANDLE OBJECTS
This section lists the methods and attributes associated with DBI
statement handles.
Statement Handle Methods
The DBI defines the following methods for use on DBI statement handles:
"bind_param"
$rc = $sth->bind_param($p_num, $bind_value) or die $sth->errstr;
$rv = $sth->bind_param($p_num, $bind_value, \%attr) or ...
$rv = $sth->bind_param($p_num, $bind_value, $bind_type) or ...
The "bind_param" method can be used to bind a value with a
placeholder embedded in the prepared statement. Placeholders are
indicated with question mark character ("?"). For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
$sth->bind_param(1, "John%"); # placeholders are numbered from 1
$sth->execute;
DBI::dump_results($sth);
Note that the "?" is not enclosed in quotation marks, even when the
placeholder represents a string. Some drivers also allow
placeholders like ":"*name* and ":"*n* (e.g., ":1", ":2", and so on)
in addition to "?", but their use is not portable. Undefined bind
values or "undef" can be used to indicate null values.
Some drivers do not support placeholders.
With most drivers, placeholders can't be used for any element of a
statement that would prevent the database server from validating the
statement and creating a query execution plan for it. For example:
"SELECT name, age FROM ?" # wrong (will probably fail)
"SELECT name, ? FROM people" # wrong (but may not 'fail')
Also, placeholders can only represent single scalar values. For
example, the following statement won't work as expected for more
than one value:
"SELECT name, age FROM people WHERE name IN (?)" # wrong
Data Types for Placeholders
The "\%attr" parameter can be used to hint at the data type the
placeholder should have. Typically, the driver is only interested in
knowing if the placeholder should be bound as a number or a string.
$sth->bind_param(1, $value, { TYPE => SQL_INTEGER });
As a short-cut for this common case, the data type can be passed
directly, in place of the "\%attr" hash reference. This example is
equivalent to the one above:
$sth->bind_param(1, $value, SQL_INTEGER);
The "TYPE" value indicates the standard (non-driver-specific) type
for this parameter. To specify the driver-specific type, the driver
may support a driver-specific attribute, such as "{ ora_type => 97
}". The data type for a placeholder cannot be changed after the
first "bind_param" call. However, it can be left unspecified, in
which case it defaults to the previous value.
The SQL_INTEGER and other related constants can be imported using
use DBI qw(:sql_types);
See "DBI Constants" for more information.
Perl only has string and number scalar data types. All database
types that aren't numbers are bound as strings and must be in a
format the database will understand.
As an alternative to specifying the data type in the "bind_param"
call, you can let the driver pass the value as the default type
("VARCHAR"). You can then use an SQL function to convert the type
within the statement. For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The "CONVERT" function used here is just an example. The actual
function and syntax will vary between different databases and is
non-portable.
See also "Placeholders and Bind Values" for more information.
"bind_param_inout"
$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) or die $sth->errstr;
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr) or ...
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...
This method acts like "bind_param", but also enables values to be
updated by the statement. The statement is typically a call to a
stored procedure. The $bind_value must be passed as a reference to
the actual value to be used.
Note that unlike "bind_param", the $bind_value variable is not read
when "bind_param_inout" is called. Instead, the value in the
variable is read at the time "execute" is called.
The additional $max_len parameter specifies the minimum amount of
memory to allocate to $bind_value for the new value. If the value
returned from the database is too big to fit, then the execution
should fail. If unsure what value to use, pick a generous length,
i.e., a length larger than the longest value that would ever be
returned. The only cost of using a larger value than needed is
wasted memory.
It is expected that few drivers will support this method. The only
driver currently known to do so is DBD::Oracle (DBD::ODBC may
support it in a future release). Therefore it should not be used for
database independent applications.
Undefined values or "undef" are used to indicate null values. See
also "Placeholders and Bind Values" for more information.
"bind_param_array"
$rc = $sth->bind_param_array($p_num, $array_ref_or_value)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)
The "bind_param_array" method is used to bind an array of values to
a placeholder embedded in the prepared statement which is to be
executed with "execute_array". For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
$sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
$sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
$sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
my @tuple_status;
$sth->execute_array( { ArrayTupleStatus => \@tuple_status } );
The %attr argument is the same as defined for "bind_param". Refer to
"bind_param" for general details on using placeholders.
Each array bound to the statement must have the same number of
elements. Some drivers may define a method attribute to relax this
safety check.
Scalar values, including "undef", may also be bound by
"bind_param_array". In which case the same value will be used for
each "execute" call. Driver-specific implementations may behave
differently, e.g., when binding to a stored procedure call, some
databases permit mixing scalars and arrays as arguments.
The default implementation provided by DBI (for drivers that have
not implemented array binding) is to iteratively "execute" for each
parameter tuple provided in the bound arrays. Drivers may provide
more optimized implementations using whatever bulk operation support
the database API provides. The default driver behaviour should match
the default DBI behaviour, but always consult your driver
documentation as there may be driver specific issues to consider.
Note that the default implementation currently only supports
non-data returning statements. Also, "bind_param_array" and
"bind_param" cannot be mixed in the same statement execution, and
"bind_param_array" must be used with "execute_array"; using
"bind_param_array" will have no effect for "execute".
The "bind_param_array" method was added in DBI 1.22.
"execute"
$rv = $sth->execute or die $sth->errstr;
$rv = $sth->execute(@bind_values) or die $sth->errstr;
Perform whatever processing is necessary to execute the prepared
statement. An "undef" is returned if an error occurs. A successful
"execute" always returns true regardless of the number of rows
affected, even if it's zero (see below). It is always important to
check the return status of "execute" (and most other DBI methods)
for errors if you're not using "RaiseError".
For a *non*-"SELECT" statement, "execute" returns the number of rows
affected, if known. If no rows were affected, then "execute" returns
"0E0", which Perl will treat as 0 but will regard as true. Note that
it is *not* an error for no rows to be affected by a statement. If
the number of rows affected is not known, then "execute" returns -1.
For "SELECT" statements, execute simply "starts" the query within
the database engine. Use one of the fetch methods to retrieve the
data after calling "execute". The "execute" method does *not* return
the number of rows that will be returned by the query (because most
databases can't tell in advance), it simply returns a true value.
If any arguments are given, then "execute" will effectively call
"bind_param" for each value before executing the statement. Values
bound in this way are usually treated as "SQL_VARCHAR" types unless
the driver can determine the correct type (which is rare), or unless
"bind_param" (or "bind_param_inout") has already been used to
specify the type.
If execute() is called on a statement handle that's still active
($sth->{Active} is true) then it should effectively call finish() to
tidy up the previous execution results before starting this new
execution.
"execute_array"
$rv = $sth->execute_array(\%attr) or die $sth->errstr;
$rv = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;
Execute the prepared statement for each parameter tuple provided
either in the @bind_values, or by prior calls to "bind_param_array".
An "undef" is returned if an error occurs. A successful
"execute_array" always returns true regardless of the number of rows
affected, even if it's zero (see below). It is always important to
check the return status of "execute_array" (and most other DBI
methods) for errors.
Parameters may be supplied either by prior calls to
"bind_param_array", or in the @bind_values argument. The values
supplied may be either scalars, or arrayrefs. See "bind_param_array"
for details.
The supplied "\%attr" hashref currently supports only the
"ArrayTupleStatus" attribute, which should specify an arrayref to
receive the status of each parameter tuple bound to the statement.
For parameter tuples which are successfully executed, the element at
the same ordinal position in the status array will return the
resulting rowcount.
For example:
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES (?, ?)");
my @tuple_status;
$sth->execute_array(
{ ArrayTupleStatus => \@tuple_status },
\@first_names,
\@last_names,
);
If a parameter tuple causes an error, the associated status array
element will be set to an arrayref of [ $sth->err, $sth->errstr ]
returned by the failed execution. If any tuple returns an error,
"execute_array" will return "undef" after it has executed all the
parameter tuples. In that case, the application should inspect the
status array to determine which parameter tuples failed.
If no "ArrayTupleStatus" is provided, "execute_array" will return
"undef" on the first occurance of a parameter tuple causing an
error. [XXX This may change as it doesn't match the behaviour of
drivers which use bulk operation API to ship the data to the
server.]
If all parameter tuples are successfully executed, "execute_array"
returns the sum of the number of rows affected by all the parameter
tuples, if known. If no rows were affected, then "execute" returns
"0E0", which Perl will treat as 0 but will regard as true. Note that
it is *not* an error for no rows to be affected by a statement. If
the number of rows affected is not known, then "execute_array" may
return a negative number. Applications should not rely on the
returned value to indicate actual total rowcounts, but use the
"ArrayTupleStatus" and explicitly inspect each returned element of
the status array.
Support for data returning statements is driver-specific and subject
to change. At present, the default implementation provided by DBI
only supports non-data returning statements.
If any @bind_values are given, then "execute_array" will effectively
call "bind_param_array" for each value before executing the
statement. Values bound in this way are usually treated as
"SQL_VARCHAR" types unless the driver can determine the correct type
(which is rare), or unless "bind_param", "bind_param_inout",
"bind_param_array", or "bind_param_inout_array" has already been
used to specify the type.
Transaction semantics using array binding are driver and database
specific. If "AutoCommit" is on, the default DBI implementation will
cause each parameter tuple to be inidividually committed (or rolled
back in the event of an error). If "AutoCommit" is off, the
application is responsible for explicitly committing the entire set
of bound parameter tuples. Note that different drivers and databases
may have different behaviours when some parameter tuples cause
failures. In some cases, the driver or database may automatically
rollback the effect of all prior parameter tuples that succeeded in
the transaction; other drivers or databases may retain the effect of
prior successfully executed parameter tuples. Be sure to check your
driver and database for its specific behaviour.
Note that, in general, performance will usually be better with
"AutoCommit" turned off, and using explicit "commit" after each
"execute_array" call.
The "execute_aray" method was added in DBI 1.22.
"fetchrow_arrayref"
$ary_ref = $sth->fetchrow_arrayref;
$ary_ref = $sth->fetch; # alias
Fetches the next row of data and returns a reference to an array
holding the field values. Null fields are returned as "undef" values
in the array. This is the fastest way to fetch data, particularly if
used with "$sth->bind_columns".
If there are no more rows or if an error occurs, then
"fetchrow_arrayref" returns an "undef". You should check "$sth->err"
afterwards (or use the "RaiseError" attribute) to discover if the
"undef" returned was due to an error.
Note that the same array reference is returned for each fetch, so
don't store the reference and then use it after a later fetch. Also,
the elements of the array are also reused for each row, so take care
if you want to take a reference to an element. See also
"bind_columns".
"fetchrow_array"
@ary = $sth->fetchrow_array;
An alternative to "fetchrow_arrayref". Fetches the next row of data
and returns it as a list containing the field values. Null fields
are returned as "undef" values in the list.
If there are no more rows or if an error occurs, then
"fetchrow_array" returns an empty list. You should check "$sth->err"
afterwards (or use the "RaiseError" attribute) to discover if the
empty list returned was due to an error.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in a
scalar context, an "undef" is returned if there are no more rows or
if an error occurred. That "undef" can't be distinguished from an
"undef" returned because the first field value was NULL. For these
reasons you should exercise some caution if you use "fetchrow_array"
in a scalar context.
"fetchrow_hashref"
$hash_ref = $sth->fetchrow_hashref;
$hash_ref = $sth->fetchrow_hashref($name);
An alternative to "fetchrow_arrayref". Fetches the next row of data
and returns it as a reference to a hash containing field name and
field value pairs. Null fields are returned as "undef" values in the
hash.
If there are no more rows or if an error occurs, then
"fetchrow_hashref" returns an "undef". You should check "$sth->err"
afterwards (or use the "RaiseError" attribute) to discover if the
"undef" returned was due to an error.
The optional $name parameter specifies the name of the statement
handle attribute. For historical reasons it defaults to ""NAME"",
however using either ""NAME_lc"" or ""NAME_uc"" is recomended for
portability.
The keys of the hash are the same names returned by "$sth->{$name}".
If more than one field has the same name, there will only be one
entry in the returned hash for those fields.
Because of the extra work "fetchrow_hashref" and Perl have to
perform, it is not as efficient as "fetchrow_arrayref" or
"fetchrow_array".
Currently, a new hash reference is returned for each row. *This will
change* in the future to return the same hash ref each time, so
don't rely on the current behaviour.
"fetchall_arrayref"
$tbl_ary_ref = $sth->fetchall_arrayref;
$tbl_ary_ref = $sth->fetchall_arrayref( $slice );
$tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
The "fetchall_arrayref" method can be used to fetch all the data to
be returned from a prepared and executed statement handle. It
returns a reference to an array that contains one reference per row.
If there are no rows to return, "fetchall_arrayref" returns a
reference to an empty array. If an error occurs, "fetchall_arrayref"
returns the data fetched thus far, which may be none. You should
check "$sth->err" afterwards (or use the "RaiseError" attribute) to
discover if the data is complete or was truncated due to an error.
If $slice is an array reference, "fetchall_arrayref" uses
"fetchrow_arrayref" to fetch each row as an array ref. If the
parameter array is not empty then it is used as a slice to select
individual columns by perl array index number (starting at 0, unlike
column and parameter numbers which start at 1).
With no parameters, or if $slice is undefined, "fetchall_arrayref"
acts as if passed an empty array ref.
If $slice is a hash reference, "fetchall_arrayref" uses
"fetchrow_hashref" to fetch each row as a hash reference. If the
$slice hash is empty then fetchrow_hashref() is simply called in a
tight loop and the keys in the hashes have whatever name lettercase
is returned by default from fetchrow_hashref. (See
"FetchHashKeyName" attribute.) If the $slice hash is not empty, then
it is used as a slice to select individual columns by name. The
values of the hash should be set to 1. The key names of the returned
hashes match the letter case of the names in the parameter hash,
regardless of the "FetchHashKeyName" attribute.
For example, to fetch just the first column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([0]);
To fetch the second to last and last column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);
To fetch all fields of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({});
To fetch only the fields called "foo" and "bar" of every row as a
hash ref (with keys named "foo" and "BAR"):
$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });
The first two examples return a reference to an array of array refs.
The third and forth return a reference to an array of hash refs.
If $max_rows is defined and greater than or equal to zero then it is
used to limit the number of rows fetched before returning.
fetchall_arrayref() can then be called again to fetch more rows.
This is especially useful when you need the better performance of
fetchall_arrayref() but don't have enough memory to fetch and return
all the rows in one go. Here's an example:
my $rows = []; # cache for batches of rows
while( my $row = ( shift(@$rows) || # get row from cache, or reload cache:
shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]) )
) {
...
}
That is the fastest way to fetch and process lots of rows using the
DBI.
"fetchall_hashref"
$hash_ref = $sth->fetchall_hashref($key_field);
The "fetchall_hashref" method can be used to fetch all the data to
be returned from a prepared and executed statement handle. It
returns a reference to a hash that contains, at most, one entry per
row.
If there are no rows to return, "fetchall_hashref" returns a
reference to an empty hash. If an error occurs, "fetchall_hashref"
returns the data fetched thus far, which may be none. You should
check "$sth->err" afterwards (or use the "RaiseError" attribute) to
discover if the data is complete or was truncated due to an error.
The $key_field parameter provides the name of the field that holds
the value to be used for the key for the returned hash. For example:
$dbh->{FetchHashKeyName} = 'NAME_lc';
$sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
$hash_ref = $sth->fetchall_hashref('id');
print "Name for id 42 is $hash_ref->{42}->{name}\n";
The $key_field parameter can also be specified as an integer column
number (counting from 1). If $key_field doesn't match any column in
the statement, as a name first then as a number, then an error is
returned.
This method is normally used only where the key field value for each
row is unique. If multiple rows are returned with the same value for
the key field then later rows overwrite earlier ones.
"finish"
$rc = $sth->finish;
Indicates that no more data will be fetched from this statement
handle before it is either executed again or destroyed. The "finish"
method is rarely needed, but can sometimes be helpful in very
specific situations to allow the server to free up resources (such
as sort buffers).
When all the data has been fetched from a "SELECT" statement, the
driver should automatically call "finish" for you. So you should
*not* normally need to call it explicitly *except* when you know
that you've not fetched all the data from a statement handle. The
most common example is when you only want to fetch one row, but in
that case the "selectrow_*" methods may be better anyway. Adding
calls to "finish" after each fetch loop is a common mistake, don't
do it, it can mask genuine problems like uncaught fetch errors.
Consider a query like:
SELECT foo FROM table WHERE bar=? ORDER BY foo
where you want to select just the first (smallest) "foo" value from
a very large table. When executed, the database server will have to
use temporary buffer space to store the sorted rows. If, after
executing the handle and selecting one row, the handle won't be
re-executed for some time and won't be destroyed, the "finish"
method can be used to tell the server that the buffer space can be
freed.
Calling "finish" resets the "Active" attribute for the statement. It
may also make some statement handle attributes (such as "NAME" and
"TYPE") unavailable if they have not already been accessed (and thus
cached).
The "finish" method does not affect the transaction status of the
database connection. It has nothing to do with transactions. It's
mostly an internal "housekeeping" method that is rarely needed. See
also "disconnect" and the "Active" attribute.
The "finish" method should have been called "cancel_select".
"rows"
$rv = $sth->rows;
Returns the number of rows affected by the last row affecting
command, or -1 if the number of rows is not known or not available.
Generally, you can only rely on a row count after a *non*-"SELECT"
"execute" (for some specific operations like "UPDATE" and "DELETE"),
or after fetching all the rows of a "SELECT" statement.
For "SELECT" statements, it is generally not possible to know how
many rows will be returned except by fetching them all. Some drivers
will return the number of rows the application has fetched so far,
but others may return -1 until all rows have been fetched. So use of
the "rows" method or $DBI::rows with "SELECT" statements is not
recommended.
One alternative method to get a row count for a "SELECT" is to
execute a "SELECT COUNT(*) FROM ..." SQL statement with the same
"..." as your query and then fetch the row count from that.
"bind_col"
$rc = $sth->bind_col($column_number, \$var_to_bind);
$rc = $sth->bind_col($column_number, \$var_to_bind, \%attr );
Binds an output column (field) of a "SELECT" statement to a Perl
variable. See "bind_columns" below for an example. Note that column
numbers count up from 1.
Whenever a row is fetched from the database, the corresponding Perl
variable is automatically updated. There is no need to fetch and
assign the values manually. The binding is performed at a very low
level using Perl aliasing so there is no extra copying taking place.
This makes using bound variables very efficient.
For maximum portability between drivers, "bind_col" should be called
after "execute". This restriction may be removed in a later version
of the DBI.
You do not need to bind output columns in order to fetch data, but
it can be useful for some applications which need either maximum
performance or greater clarity of code. The "bind_param" method
performs a similar but opposite function for input variables.
"bind_columns"
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls "bind_col" for each column of the "SELECT" statement. The
"bind_columns" method will die if the number of references does not
match the number of fields.
For maximum portability between drivers, "bind_columns" should be
called after "execute".
For example:
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
$sth->execute;
my ($region, $sales);
# Bind Perl variables to columns:
$rv = $sth->bind_columns(\$region, \$sales);
# you can also use Perl's \(...) syntax (see perlref docs):
# $sth->bind_columns(\($region, $sales));
# Column binding is the most efficient way to fetch data
while ($sth->fetch) {
print "$region: $sales\n";
}
For compatibility with old scripts, the first parameter will be
ignored if it is "undef" or a hash reference.
Here's a more fancy example that binds columns to the values
*inside* a hash (thanks to H.Merijn Brand):
$sth->execute;
my %row;
$sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
while ($sth->fetch) {
print "$row{region}: $row{sales}\n";
}
"dump_results"
$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);
Fetches all the rows from $sth, calls "DBI::neat_list" for each row,
and prints the results to $fh (defaults to "STDOUT") separated by
$lsep (default "\n"). $fsep defaults to ", " and $maxlen defaults to
35.
This method is designed as a handy utility for prototyping and
testing queries. Since it uses "neat_list" to format and edit the
string for reading by humans, it is not recomended for data transfer
applications.
Statement Handle Attributes
This section describes attributes specific to statement handles. Most of
these attributes are read-only.
Changes to these statement handle attributes do not affect any other
existing or future statement handles.
Attempting to set or get the value of an unknown attribute is fatal,
except for private driver specific attributes (which all have names
starting with a lowercase letter).
Example:
... = $h->{NUM_OF_FIELDS}; # get/read
Note that some drivers cannot provide valid values for some or all of
these attributes until after "$sth->execute" has been called.
See also "finish" to learn more about the effect it may have on some
attributes.
"NUM_OF_FIELDS" (integer, read-only)
Number of fields (columns) in the data the prepared statement may
return. Statements that don't return rows of data, like "DELETE" and
"CREATE" set "NUM_OF_FIELDS" to 0.
"NUM_OF_PARAMS" (integer, read-only)
The number of parameters (placeholders) in the prepared statement.
See SUBSTITUTION VARIABLES below for more details.
"NAME" (array-ref, read-only)
Returns a reference to an array of field names for each column. The
names may contain spaces but should not be truncated or have any
trailing space. Note that the names have the letter case (upper,
lower or mixed) as returned by the driver being used. Portable
applications should use "NAME_lc" or "NAME_uc".
print "First column name: $sth->{NAME}->[0]\n";
"NAME_lc" (array-ref, read-only)
Like "NAME" but always returns lowercase names.
"NAME_uc" (array-ref, read-only)
Like "NAME" but always returns uppercase names.
"NAME_hash" (hash-ref, read-only)
"NAME_lc_hash" (hash-ref, read-only)
"NAME_uc_hash" (hash-ref, read-only)
The "NAME_hash", "NAME_lc_hash", and "NAME_uc_hash" attributes
return column name information as a reference to a hash.
The keys of the hash are the names of the columns. The letter case
of the keys corresponds to the letter case returned by the "NAME",
"NAME_lc", and "NAME_uc" attributes respectively (as described
above).
The value of each hash entry is the perl index number of the
corresponding column (counting from 0). For example:
$sth = $dbh->prepare("select Id, Name from table");
$sth->execute;
@row = $sth->fetchrow_array;
print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";
"TYPE" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
The value indicates the data type of the corresponding column.
The values correspond to the international standards (ANSI X3.135
and ISO/IEC 9075) which, in general terms, means ODBC.
Driver-specific types that don't exactly match standard types should
generally return the same values as an ODBC driver supplied by the
makers of the database. That might include private type numbers in
ranges the vendor has officially registered with the ISO working
group:
ftp://sqlstandards.org/SC32/SQL_Registry/
Where there's no vendor-supplied ODBC driver to be compatible with,
the DBI driver can use type numbers in the range that is now
officially reserved for use by the DBI: -9999 to -9000.
All possible values for "TYPE" should have at least one entry in the
output of the "type_info_all" method (see "type_info_all").
"PRECISION" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
For non-numeric columns, the value generally refers to either the
maximum length or the defined length of the column. For numeric
columns, the value refers to the maximum number of significant
digits used by the data type (without considering a sign character
or decimal point). Note that for floating point types (REAL, FLOAT,
DOUBLE), the "display size" can be up to 7 characters greater than
the precision. (for the sign + decimal point + the letter E + a sign
+ 2 or 3 digits).
"SCALE" (array-ref, read-only)
Returns a reference to an array of integer values for each column.
NULL ("undef") values indicate columns where scale is not
applicable.
"NULLABLE" (array-ref, read-only)
Returns a reference to an array indicating the possibility of each
column returning a null. Possible values are 0 (or an empty string)
= no, 1 = yes, 2 = unknown.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
"CursorName" (string, read-only)
Returns the name of the cursor associated with the statement handle,
if available. If not available or if the database driver does not
support the "where current of ..." SQL syntax, then it returns
"undef".
"Database" (dbh, read-only)
Returns the parent $dbh of the statement handle.
"ParamValues" (hash ref, read-only)
Returns a reference to a hash containing the values currently bound
to placeholders. The keys of the hash are the 'names' of the
placeholders, typically integers starting at 1. Returns undef if not
supported by the driver.
See "ShowErrorStatement" for an example of how this is used.
If the driver supports "ParamValues" but no values have been bound
yet then the driver should return a hash with placeholders names in
the keys but all the values undef, but some drivers may return a ref
to an empty hash.
It is possible that the values in the hash returned by "ParamValues"
are not exactly the same as those passed to bind_param() or
execute(). The driver may have modified the values in some way based
on the TYPE the value was bound with. For example a floating point
value bound as an SQL_INTEGER type may be returned as an integer.
It is also possible that the keys in the hash returned by
"ParamValues" are not exactly the same as those implied by the
prepared statement. For example, DBD::Oracle translates '"?"'
placeholders into '":pN"' where N is a sequence number starting at
1.
The "ParamValues" attribute was added in DBI 1.28.
"Statement" (string, read-only)
Returns the statement string passed to the "prepare" method.
"RowsInCache" (integer, read-only)
If the driver supports a local row cache for "SELECT" statements,
then this attribute holds the number of un-fetched rows in the
cache. If the driver doesn't, then it returns "undef". Note that
some drivers pre-fetch rows on execute, whereas others wait till the
first fetch.
See also the "RowCacheSize" database handle attribute.
OTHER METHODS
"install_method"
DBD::Foo::db->install_method($method_name, \%attr);
Installs the driver-private method named by $method_name into the
DBI method dispatcher so it can be called directly, avoiding the
need to use the func() method.
It is called as a static method on the driver class to which the
method belongs. The method name must begin with the corresponding
registered driver-private prefix. For example, for DBD::Oracle
$method_name must being with '"ora_"', and for DBD::AnyData it must
begin with '"ad_"'.
The attributes can be used to provide fine control over how the DBI
dispatcher handles the dispatching of the method. However, at this
point, it's undocumented and very liable to change. (Volunteers to
polish up and document the interface are very welcome to get in
touch via dbi-dev@perl.org)
Methods installed using install_method default to the standard error
handling behaviour for DBI methods: clearing err and errstr before
calling the method, and checking for errors to trigger RaiseError
etc. on return. This differs from the default behaviour of func().
FURTHER INFORMATION
Catalog Methods
An application can retrieve metadata information from the DBMS by
issuing appropriate queries on the views of the Information Schema.
Unfortunately, "INFORMATION_SCHEMA" views are seldom supported by the
DBMS. Special methods (catalog methods) are available to return result
sets for a small but important portion of that metadata:
column_info
foreign_key_info
primary_key_info
table_info
All catalog methods accept arguments in order to restrict the result
sets. Passing "undef" to an optional argument does not constrain the
search for that argument. However, an empty string ('') is treated as a
regular search criteria and will only match an empty value.
Note: SQL/CLI and ODBC differ in the handling of empty strings. An empty
string will not restrict the result set in SQL/CLI.
Most arguments in the catalog methods accept only *ordinary values*,
e.g. the arguments of "primary_key_info()". Such arguments are treated
as a literal string, i.e. the case is significant and quote characters
are taken literally.
Some arguments in the catalog methods accept *search patterns* (strings
containing '_' and/or '%'), e.g. the $table argument of "column_info()".
Passing '%' is equivalent to leaving the argument "undef".
Caveat: The underscore ('_') is valid and often used in SQL identifiers.
Passing such a value to a search pattern argument may return more rows
than expected! To include pattern characters as literals, they must be
preceded by an escape character which can be achieved with
$esc = $dbh->get_info( 14 ); # SQL_SEARCH_PATTERN_ESCAPE
$search_pattern =~ s/([_%])/$esc$1/g;
The ODBC and SQL/CLI specifications define a way to change the default
behaviour described above: All arguments (except *list value arguments*)
are treated as *identifier* if the "SQL_ATTR_METADATA_ID" attribute is
set to "SQL_TRUE". *Quoted identifiers* are very similar to *ordinary
values*, i.e. their body (the string within the quotes) is interpreted
literally. *Unquoted identifiers* are compared in UPPERCASE.
The DBI (currently) does not support the "SQL_ATTR_METADATA_ID"
attribute, i.e. it behaves like an ODBC driver where
"SQL_ATTR_METADATA_ID" is set to "SQL_FALSE".
Transactions
Transactions are a fundamental part of any robust database system. They
protect against errors and database corruption by ensuring that sets of
related changes to the database take place in atomic (indivisible,
all-or-nothing) units.
This section applies to databases that support transactions and where
"AutoCommit" is off. See "AutoCommit" for details of using "AutoCommit"
with various types of databases.
The recommended way to implement robust transactions in Perl
applications is to use "RaiseError" and "eval { ... }" (which is very
fast, unlike "eval "...""). For example:
$dbh->{AutoCommit} = 0; # enable transactions, if possible
$dbh->{RaiseError} = 1;
eval {
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
$dbh->commit; # commit the changes if we get this far
};
if ($@) {
warn "Transaction aborted because $@";
$dbh->rollback; # undo the incomplete changes
# add other application on-error-clean-up code here
}
If the "RaiseError" attribute is not set, then DBI calls would need to
be manually checked for errors, typically like this:
$h->method(@args) or die $h->errstr;
With "RaiseError" set, the DBI will automatically "die" if any DBI
method call on that handle (or a child handle) fails, so you don't have
to test the return value of each method call. See "RaiseError" for more
details.
A major advantage of the "eval" approach is that the transaction will be
properly rolled back if *any* code (not just DBI calls) in the inner
application dies for any reason. The major advantage of using the
"$h->{RaiseError}" attribute is that all DBI calls will be checked
automatically. Both techniques are strongly recommended.
After calling "commit" or "rollback" many drivers will not let you fetch
from a previously active "SELECT" statement handle that's a child of the
same database handle. A typical way round this is to connect the the
database twice and use one connection for "SELECT" statements.
See "AutoCommit" and "disconnect" for other important information about
transactions.
Handling BLOB / LONG / Memo Fields
Many databases support "blob" (binary large objects), "long", or similar
datatypes for holding very long strings or large amounts of binary data
in a single field. Some databases support variable length long values
over 2,000,000,000 bytes in length.
Since values of that size can't usually be held in memory, and because
databases can't usually know in advance the length of the longest long
that will be returned from a "SELECT" statement (unlike other data
types), some special handling is required.
In this situation, the value of the "$h->{LongReadLen}" attribute is
used to determine how much buffer space to allocate when fetching such
fields. The "$h->{LongTruncOk}" attribute is used to determine how to
behave if a fetched value can't fit into the buffer.
When trying to insert long or binary values, placeholders should be used
since there are often limits on the maximum size of an "INSERT"
statement and the "quote" method generally can't cope with binary data.
See "Placeholders and Bind Values".
Simple Examples
Here's a complete example program to select and fetch some data:
my $data_source = "dbi::DriverName:db_name";
my $dbh = DBI->connect($data_source, $user, $password)
or die "Can't connect to $data_source: $DBI::errstr";
my $sth = $dbh->prepare( q{
SELECT name, phone
FROM mytelbook
}) or die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
or die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "Field names: @{ $sth->{NAME} }\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
die $sth->errstr if $sth->err;
$dbh->disconnect;
Here's a complete example program to insert some data from a file. (This
example uses "RaiseError" to avoid needing to check each call).
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
RaiseError => 1, AutoCommit => 0
});
my $sth = $dbh->prepare( q{
INSERT INTO table (name, phone) VALUES (?, ?)
});
open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
while (<FH>) {
chomp;
my ($name, $phone) = split /,/;
$sth->execute($name, $phone);
}
close FH;
$dbh->commit;
$dbh->disconnect;
Here's how to convert fetched NULLs (undefined values) into empty
strings:
while($row = $sth->fetchrow_arrayref) {
# this is a fast and simple way to deal with nulls:
foreach (@$row) { $_ = '' unless defined }
print "@$row\n";
}
The "q{...}" style quoting used in these examples avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
"qq{...}" operator if you want to interpolate variables into the string.
See "Quote and Quote-like Operators" in perlop for more details.
Threads and Thread Safety
Perl 5.7 and later support a new threading model called iThreads. (The
old and fatally flawed "5.005 style" threads are not supported by the
DBI.)
In the iThreads model each thread has it's own copy of the perl
interpreter. When a new thread is created the original perl interpreter
is 'cloned' to create a new copy for the new thread.
If the DBI and drivers are loaded and handles created before the thread
is created then it will get a cloned copy of the DBI, the drivers and
the handles.
However, the internal pointer data within the handles will refer to the
DBI and drivers in the original interpreter. Using those handles in the
new interpreter thread is not safe, so the DBI detects this and croaks
on any method call using handles that don't belong to the current thread
(except for DESTROY).
Because of this (possibly temporary) restriction, newly created threads
must make their own connctions to the database. Handles can't be shared
across threads.
But BEWARE, some underlying database APIs (the code the DBD driver uses
to talk to the database, often supplied by the database vendor) are not
thread safe. If it's not thread safe, then allowing more than one thread
to enter the code at the same time may cause subtle/serious problems. In
some cases allowing more than one thread to enter the code, even if
*not* at the same time, can cause problems. You have been warned.
Using DBI with perl threads is not yet recommended for production
environments.
Signal Handling and Canceling Operations
The first thing to say is that signal handling in Perl is currently
*not* safe. There is always a small risk of Perl crashing and/or core
dumping when, or after, handling a signal. (The risk was reduced with
5.004_04 but is still present.)
The two most common uses of signals in relation to the DBI are for
canceling operations when the user types Ctrl-C (interrupt), and for
implementing a timeout using "alarm()" and $SIG{ALRM}.
To assist in implementing these operations, the DBI provides a "cancel"
method for statement handles. The "cancel" method should abort the
current operation and is designed to be called from a signal handler.
However, it must be stressed that: a) few drivers implement this at the
moment (the DBI provides a default method that just returns "undef");
and b) even if implemented, there is still a possibility that the
statement handle, and possibly the parent database handle, will not be
usable afterwards.
If "cancel" returns true, then it has successfully invoked the database
engine's own cancel function. If it returns false, then "cancel" failed.
If it returns "undef", then the database engine does not have cancel
implemented.
Subclassing the DBI
DBI can be subclassed and extended just like any other object oriented
module. Before we talk about how to do that, it's important to be clear
about how the DBI classes and how they work together.
By default "$dbh = DBI->connect(...)" returns a $dbh blessed into the
"DBI::db" class. And the "$dbh->prepare" method returns an $sth blessed
into the "DBI::st" class (actually it simply changes the last four
characters of the calling handle class to be "::st").
The leading '"DBI"' is known as the 'root class' and the extra '"::db"'
or '"::st"' are the 'handle type suffixes'. If you want to subclass the
DBI you'll need to put your overriding methods into the appropriate
classes. For example, if you want to use a root class of "MySubDBI" and
override the do(), prepare() and execute() methods, then your do() and
prepare() methods should be in the "MySubDBI::db" class and the
execute() method should be in the "MySubDBI::st" class.
To setup the inheritance hierarchy the @ISA variable in "MySubDBI::db"
should include "DBI::db" and the @ISA variable in "MySubDBI::st" should
include "DBI::st". The "MySubDBI" root class itself isn't currently used
for anything visible and so, apart from setting @ISA to include "DBI",
it should be left empty.
So, having put your overriding methods into the right classes, and setup
the inheritance hierarchy, how do you get the DBI to use them? You have
two choices, either a static method call using the name of your
subclass:
$dbh = MySubDBI->connect(...);
or specifying a "RootClass" attribute:
$dbh = DBI->connect(..., { RootClass => 'MySubDBI' });
The only difference between the two is that using an explicit RootClass
attribute will make the DBI automatically attempt to load a module by
that name if the class doesn't exist.
If both forms are used then the attribute takes precedence.
The when subclassing is being used then, after a successful new connect,
the DBI->connect method automatically calls:
$dbh->connected($dsn, $user, $pass, \%attr);
The default method does nothing. The call is made just to simplify any
post-connection setup that your subclass may want to perform. If your
subclass supplies a connected method, it should be part of the
MySubDBI::db package.
Here's a brief example of a DBI subclass. A more thorough example can be
found in t/subclass.t in the DBI distribution.
package MySubDBI;
use strict;
use DBI;
use vars qw(@ISA);
@ISA = qw(DBI);
package MySubDBI::db;
use vars qw(@ISA);
@ISA = qw(DBI::db);
sub prepare {
my ($dbh, @args) = @_;
my $sth = $dbh->SUPER::prepare(@args)
or return;
$sth->{private_mysubdbi_info} = { foo => 'bar' };
return $sth;
}
package MySubDBI::st;
use vars qw(@ISA);
@ISA = qw(DBI::st);
sub fetch {
my ($sth, @args) = @_;
my $row = $sth->SUPER::fetch(@args)
or return;
do_something_magical_with_row_data($row)
or return $sth->set_err(1234, "The magic failed", undef, "fetch");
return $row;
}
When calling a SUPER::method that returns a handle, be careful to check
the return value before trying to do other things with it in your
overridden method. This is especially important if you want to set a
hash attribute on the handle, as Perl's autovivification will bite you
by (in)conveniently creating an unblessed hashref, which your method
will then return with usually baffling results later on. It's best to
check right after the call and return undef immediately on error, just
like DBI would and just like the example above.
If your method needs to record an error it should call the set_err()
method with the error code and error string, as shown in the example
above. The error code and error string will be recorded in the handle
and available via "$h->err" and $DBI::errstr etc. The set_err() method
always returns an undef or empty list as approriate. Since your method
should nearly always return an undef or empty list as soon as an error
is detected it's handy to simply return what set_err() returns, as shown
in the example above.
If the handle has "RaiseError", "PrintError", or "HandleError" etc. set
then the set_err() method will honour them. This means that if
"RaiseError" is set then set_err() won't return in the normal way but
will 'throw an exception' that can be caught with an "eval" block.
You can stash private data into DBI handles via "$h->{private_..._*}".
See the entry under "ATTRIBUTES COMMON TO ALL HANDLES" for info and
important caveats.
DEBUGGING
In addition to the "trace" method, you can enable the same trace
information by setting the "DBI_TRACE" environment variable before
starting Perl.
On Unix-like systems using a Bourne-like shell, you can do this easily
on the command line:
DBI_TRACE=2 perl your_test_script.pl
If "DBI_TRACE" is set to a non-numeric value, then it is assumed to be a
file name and the trace level will be set to 2 with all trace output
appended to that file. If the name begins with a number followed by an
equal sign ("="), then the number and the equal sign are stripped off
from the name, and the number is used to set the trace level. For
example:
DBI_TRACE=1=dbitrace.log perl your_test_script.pl
See also the "trace" method.
It can sometimes be handy to compare trace files from two different runs
of the same script. However using a tool like "diff" doesn't work well
because the trace file is full of object addresses that may differ each
run. Here's a handy little command to strip those out:
perl -pe 's/\b0x[\da-f]{6,}/0xNNNN/gi; s/\b[\da-f]{6,}/<long number>/gi'
DBI ENVIRONMENT VARIABLES
The DBI module recognizes a number of environment variables, but most of
them should not be used most of the time. It is better to be explicit
about what you are doing to avoid the need for environment variables,
especially in a web serving system where web servers are stingy about
which environment variables are available.
DBI_DSN
The DBI_DSN environment variable is used by DBI->connect if you do not
specify a data source when you issue the connect. It should have a
format such as "dbi:Driver:databasename".
DBI_DRIVER
The DBI_DRIVER environment variable is used to fill in the database
driver name in DBI->connect if the data source string starts "dbi::"
(thereby omitting the driver). If DBI_DSN omits the driver name,
DBI_DRIVER can fill the gap.
DBI_AUTOPROXY
The DBI_AUTOPROXY environment variable takes a string value that starts
"dbi:Proxy:" and is typically followed by "hostname=...;port=...". It is
used to alter the behaviour of DBI->connect. For full details, see
DBI::Proxy documentation.
DBI_USER
The DBI_USER environment variable takes a string value that is used as
the user name if the DBI->connect call is given undef (as distinct from
an empty string) as the username argument. Be wary of the security
implications of using this.
DBI_PASS
The DBI_PASS environment variable takes a string value that is used as
the password if the DBI->connect call is given undef (as distinct from
an empty string) as the password argument. Be extra wary of the security
implications of using this.
DBI_DBNAME (obsolete)
The DBI_DBNAME environment variable takes a string value that is used
only when the obsolescent style of DBI->connect (with driver name as
fourth parameter) is used, and when no value is provided for the first
(database name) argument.
DBI_TRACE
The DBI_TRACE environment variable takes an integer value that specifies
the trace level for DBI at startup. Can also be used to direct trace
output to a file. See "DEBUGGING" for more information.
PERL_DBI_DEBUG (obsolete)
An old variable that should no longer be used; equivalent to DBI_TRACE.
DBI_PROFILE
The DBI_PROFILE environment variable can be used to enable profiling of
DBI method calls. See <DBI::Profile> for more information.
DBI_PUREPERL
The DBI_PUREPERL environment variable can be used to enable the use of
DBI::PurePerl. See <DBI::PurePerl> for more information.
WARNING AND ERROR MESSAGES
Fatal Errors
Can't call method "prepare" without a package or object reference
The $dbh handle you're using to call "prepare" is probably undefined
because the preceding "connect" failed. You should always check the
return status of DBI methods, or use the "RaiseError" attribute.
Can't call method "execute" without a package or object reference
The $sth handle you're using to call "execute" is probably undefined
because the preceeding "prepare" failed. You should always check the
return status of DBI methods, or use the "RaiseError" attribute.
DBI/DBD internal version mismatch
The DBD driver module was built with a different version of DBI than
the one currently being used. You should rebuild the DBD module
under the current version of DBI.
(Some rare platforms require "static linking". On those platforms,
there may be an old DBI or DBD driver version actually embedded in
the Perl executable being used.)
DBD driver has not implemented the AutoCommit attribute
The DBD driver implementation is incomplete. Consult the author.
Can't [sg]et %s->{%s}: unrecognised attribute
You attempted to set or get an unknown attribute of a handle. Make
sure you have spelled the attribute name correctly; case is
significant (e.g., "Autocommit" is not the same as "AutoCommit").
Pure-Perl DBI
A pure-perl emulation of the DBI is included in the distribution for
people using pure-perl drivers who, for whatever reason, can't install
the compiled DBI. See DBI::PurePerl.
SEE ALSO
Driver and Database Documentation
Refer to the documentation for the DBD driver that you are using.
Refer to the SQL Language Reference Manual for the database engine that
you are using.
ODBC and SQL/CLI Standards Reference Information
More detailed information about the semantics of certain DBI methods
that are based on ODBC and SQL/CLI standards is available on-line via
microsoft.com, for ODBC, and www.jtc1sc32.org for the SQL/CLI standard:
DBI method ODBC function SQL/CLI Working Draft
---------- ------------- ---------------------
column_info SQLColumns Page 124
foreign_key_info SQLForeignKeys Page 163
get_info SQLGetInfo Page 214
primary_key_info SQLPrimaryKeys Page 254
table_info SQLTables Page 294
type_info SQLGetTypeInfo Page 239
For example, for ODBC information on SQLColumns you'd visit:
http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp
If that URL ceases to work then use the MSDN search facility at:
http://search.microsoft.com/us/dev/
and search for "SQLColumns returns" using the exact phrase option. The
link you want will probably just be called "SQLColumns" and will be part
of the Data Access SDK.
And for SQL/CLI standard information on SQLColumns you'd read page 124
of the (very large) SQL/CLI Working Draft available from:
http://www.jtc1sc32.org/sc32/jtc1sc32.nsf/Attachments/7E3B41486BD99C3488256B410064C877/$FILE/32N0744T.PDF
SQL Standards Reference Information
A hyperlinked, browsable version of the BNF syntax for SQL92 (plus
Oracle 7 SQL and PL/SQL) is available here:
http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html
A BNF syntax for SQL3 is available here:
http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf
The following links provide further useful information about SQL. Some
of these are rather dated now but may still be useful.
http://www.jcc.com/SQLPages/jccs_sql.htm
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
http://www.altavista.com/query?q=sql+tutorial
Books and Journals
Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
Programming Perl 3rd Ed. by Larry Wall, Tom Christiansen & Jon Orwant.
Learning Perl by Randal Schwartz.
Dr Dobb's Journal, November 1996.
The Perl Journal, April 1997.
Perl Modules
Index of DBI related modules available from CPAN:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
http://search.cpan.org/search?mode=doc&query=DBI
For a good comparison of RDBMS-OO mappers and some OO-RDBMS mappers
(including Class::DBI, Alzabo, and DBIx::RecordSet in the former
category and Tangram and SPOPS in the latter) see the Perl
Object-Oriented Persistence project pages at:
http://poop.sourceforge.net
A similar page for Java toolkits can be found at:
http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison
Manual Pages
perl(1), perlmod(1), perlbook(1)
Mailing List
The *dbi-users* mailing list is the primary means of communication among
users of the DBI and its related modules. For details send email to:
dbi-users-help@perl.org
There are typically between 700 and 900 messages per month. You have to
subscribe in order to be able to post. However you can opt for a
'post-only' subscription.
Mailing list archives (of variable quality) are held at:
http://www.xray.mpe.mpg.de/mailing-lists/dbi/
http://groups.yahoo.com/group/dbi-users
http://www.bitmechanic.com/mail-archives/dbi-users/
http://marc.theaimsgroup.com/?l=perl-dbi&r=1&w=2
http://www.mail-archive.com/dbi-users%40perl.org/
Assorted Related WWW Links
The DBI "Home Page":
http://dbi.perl.org/
Other DBI related links:
http://tegan.deltanet.com/~phlip/DBUIdoc.html
http://dc.pm.org/perl_db.html
http://wdvl.com/Authoring/DB/Intro/toc.html
http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
http://bumppo.net/lists/macperl/1999/06/msg00197.html
Other database related links:
http://www.jcc.com/sql_stnd.html
http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
Security, especially the "SQL Injection" attack:
http://online.securityfocus.com/infocus/1644
http://www.nextgenss.com/research/papers.html
Commercial and Data Warehouse Links
http://www.dwinfocenter.org
http://www.datawarehouse.com
http://www.datamining.org
http://www.olapcouncil.org
http://www.idwa.org
http://www.knowledgecenters.org/dwcenter.asp
Recommended Perl Programming Links
http://language.perl.com/style/
FAQ
Please also read the DBI FAQ which is installed as a DBI::FAQ module.
You can use *perldoc* to read it by executing the "perldoc DBI::FAQ"
command.
AUTHORS
DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas Dunlop,
Jonathan Leffler and others. Perl by Larry Wall and the "perl5-porters".
COPYRIGHT
The DBI module is Copyright (c) 1994-2002 Tim Bunce. Ireland. All rights
reserved.
You may distribute under the terms of either the GNU General Public
License or the Artistic License, as specified in the Perl README file.
ACKNOWLEDGEMENTS
I would like to acknowledge the valuable contributions of the many
people I have worked with on the DBI project, especially in the early
years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti,
Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael Peppler,
Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander, Forrest D
Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson, Georg
Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen, Steve
Baumgarten, Randal Schwartz, and a whole lot more.
Then, of course, there are the poor souls who have struggled through
untold and undocumented obstacles to actually implement DBI drivers.
Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan
Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo,
Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve
Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would
not be the practical reality it is today. I'm also especially grateful
to Alligator Descartes for starting work on the "Programming the Perl
DBI" book and letting me jump on board.
Much of the DBI and DBD::Oracle was developed while I was Technical
Director (CTO) of the Paul Ingram Group (www.ig.co.uk). So I'd
especially like to thank Paul for his generosity and vision in
supporting this work for many years.
TRANSLATIONS
A German translation of this manual (possibly slightly out of date) is
available, thanks to O'Reilly, at:
http://www.oreilly.de/catalog/perldbiger/
Some other translations:
http://cronopio.net/perl/ - Spanish
http://member.nifty.ne.jp/hippo2000/dbimemo.htm - Japanese
SUPPORT / WARRANTY
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.
Commercial support for Perl and the DBI, DBD::Oracle and Oraperl modules
can be arranged via The Perl Clinic. For more details visit:
http://www.perlclinic.com
For direct DBI and DBD::Oracle support, enhancement, and related work I
am available for consultancy on standard commercial terms.
TRAINING
References to DBI related training resources. No recommendation implied.
http://www.treepax.co.uk/
http://www.keller.com/dbweb/
FREQUENTLY ASKED QUESTIONS
See the DBI FAQ for a more comprehensive list of FAQs. Use the "perldoc
DBI::FAQ" command to read it.
How fast is the DBI?
To measure the speed of the DBI and DBD::Oracle code, I modified
DBD::Oracle so you can set an attribute that will cause the same row to
be fetched from the row cache over and over again (without involving
Oracle code but exercising *all* the DBI and DBD::Oracle code in the
code path for a fetch).
The results (on my lightly loaded old Sparc 10) fetching 50000 rows
using:
1 while $csr->fetch;
were: one field: 5300 fetches per cpu second (approx) ten fields: 4000
fetches per cpu second (approx)
Obviously results will vary between platforms (newer faster platforms
can reach around 50000 fetches per second), but it does give a feel for
the maximum performance: fast. By way of comparison, using the code:
1 while @row = $csr->fetchrow_array;
("fetchrow_array" is roughly the same as "ora_fetch") gives:
one field: 3100 fetches per cpu second (approx)
ten fields: 1000 fetches per cpu second (approx)
Notice the slowdown and the more dramatic impact of extra fields. (The
fields were all one char long. The impact would be even bigger for
longer strings.)
Changing that slightly to represent actually doing something in Perl
with the fetched data:
while(@row = $csr->fetchrow_array) {
$hash{++$i} = [ @row ];
}
gives: ten fields: 500 fetches per cpu second (approx)
That simple addition has *halved* the performance.
I therefore conclude that DBI and DBD::Oracle overheads are small
compared with Perl language overheads (and probably database overheads).
So, if you think the DBI or your driver is slow, try replacing your
fetch loop with just:
1 while $csr->fetch;
and time that. If that helps then point the finger at your own code. If
that doesn't help much then point the finger at the database, the
platform, the network etc. But think carefully before pointing it at the
DBI or your driver.
(Having said all that, if anyone can show me how to make the DBI or
drivers even more efficient, I'm all ears.)
Why doesn't my CGI script work right?
Read the information in the references below. Please do *not* post CGI
related questions to the *dbi-users* mailing list (or to me).
http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
http://www.boutell.com/faq/
http://www.perl.com/perl/faq/
General problems and good ideas:
Use the CGI::ErrorWrap module.
Remember that many env vars won't be set for CGI scripts.
How can I maintain a WWW connection to a database?
For information on the Apache httpd server and the "mod_perl" module see
http://perl.apache.org/
What about ODBC?
A DBD::ODBC module is available.
Does the DBI have a year 2000 problem?
No. The DBI has no knowledge or understanding of dates at all.
Individual drivers (DBD::*) may have some date handling code but are
unlikely to have year 2000 related problems within their code. However,
your application code which *uses* the DBI and DBD drivers may have year
2000 related problems if it has not been designed and written well.
See also the "Does Perl have a year 2000 problem?" section of the Perl
FAQ:
http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
OTHER RELATED WORK AND PERL MODULES
Apache::DBI by E.Mergl@bawue.de
To be used with the Apache daemon together with an embedded Perl
interpreter like "mod_perl". Establishes a database connection which
remains open for the lifetime of the HTTP daemon. This way the CGI
connect and disconnect for every database access becomes
superfluous.
JDBC Server by Stuart 'Zen' Bishop zen@bf.rmit.edu.au
The server is written in Perl. The client classes that talk to it
are of course in Java. Thus, a Java applet or application will be
able to comunicate via the JDBC API with any database that has a DBI
driver installed. The URL used is in the form
"jdbc:dbi://host.domain.etc:999/Driver/DBName". It seems to be very
similar to some commercial products, such as jdbcKona.
Remote Proxy DBD support
As of DBI 1.02, a complete implementation of a DBD::Proxy driver and
the DBI::ProxyServer are part of the DBI distribution.
SQL Parser
See also the SQL::Statement module, SQL parser and engine.
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