DBI - Database independent interface for Perl
SYNOPSIS
use DBI;
@data_sources = DBI->data_sources($driver_name);
$dbh = DBI->connect($data_source, $username, $auth);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rc = $dbh->disconnect;
$rv = $dbh->do($statement);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare($statement, \%attr);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(\%attr, @list_of_refs_to_vars_to_bind);
$rv = $sth->bind_param($param_num, $bind_value);
$rv = $sth->bind_param($param_num, $bind_value, $bind_type);
$rv = $sth->bind_param($param_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$rc = $sth->finish;
$rv = $sth->rows;
$rc = $dbh->commit;
$rc = $dbh->rollback;
$sql = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
NOTE
This is the draft DBI specification that corresponds to
the DBI version 0.91 ($Date: 1998/01/09 23:03:44 $).
The RECENT CHANGES section below has a summary of user-
visible changes and the Changes file supplied with the DBI
holds more detailed change information.
Note also that whenever the DBI changes the drivers take
some time to catch up. Recent versions of the DBI have
added many new features that may not yet be supported by
the drivers you use. Talk to the authors of those drivers
if you need the features.
Please also read the DBI FAQ which is installed as a
DBI::FAQ module so you can use perldoc to read it by
executing the perldoc DBI::FAQ command.
RECENT CHANGES
A brief summary of significant user-visible changes in
recent versions (if a recent version isn't mentioned it
simply means that there were no significant user-visible
changes in that version).
DBI 0.91 - 10th December 1997
Fixed bug in New-style DBI->connect call which was not
defaulting AutoCommit and PrintError to on.
DBI 0.86 - 16th July 1997
Added $h->{LongReadLen} and $h->{LongTruncOk}
attributes for LONGs/BLOBs. Added DBI_USER and
DBI_PASS env vars. See the connect entry elsewhere in
this document for usage. Added DBI->trace() to set
global trace level (like per-handle $h->trace).
PERL_DBI_DEBUG env var renamed DBI_TRACE (old name
still works for now). Updated docs, including commit,
rollback, AutoCommit and Transactions sections. Added
bind_param method and execute(@bind_values) to docs.
DBI 0.85 - 25th June 1997
The 'new-style connect' (see below) now defaults to
AutoCommit mode unless { AutoCommit => 0 } specified
in connect attributes (see the connect entry elsewhere
in this document ). New DBI_DSN env var default for
connect method (supersedes DBI_DRIVER).
DBI 0.84 - 20th June 1997
Added $h->{PrintError} attribute which, if set true,
causes all errors to trigger a warn(). New-style
DBI->connect call now automatically sets PrintError=1
unless { PrintError => 0 } specified in the connect
attributes (see the connect entry elsewhere in this
document ). The old-style connect with a separate
driver parameter is deprecated. Renamed $h->debug to
$h->trace() and added a trace filename arg.
Added 'new-style' driver specification syntax to the
DBI->connect data_source parameter: DBI->connect(
'dbi:driver:...', $user, $passwd); The
DBI->data_sources method should return data_source
names with the appropriate 'dbi:driver:' prefix.
DBI->connect will warn if \%attr is true but not a
hash ref. Added new fetchrow methods (fetchrow_array,
fetchrow_arrayref and fetchrow_hashref): Added the
DBI FAQ from Alligator Descartes in module form for
easy reading via "perldoc DBI::FAQ".
DESCRIPTION
The Perl DBI is a database access Application Programming
Interface (API) for the Perl Language. The DBI defines a
set of functions, variables and conventions that provide a
consistent database interface independant of the actual
database being used.
It is important to remember that the DBI is just an
interface. A thin layer of 'glue' between an application
and one or more Database Drivers. It is the drivers which
do the real work. The DBI provides a standard interface
and framework for the drivers to operate within.
This document is a work-in-progress. Although it is
incomplete it should be useful in getting started with the
DBI.
Architecture of a DBI Application
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | |S| `--------------' `-------------'
| script| |A| |w| .--------------. .-------------.
| using |--|P|--|i|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |t| `--------------' `-------------'
| API | |c|...
|methods| |h|... Other drivers
`-------' | |...
`-'
The API is the Application Perl-script (or Programming)
Interface. The call interface and variables provided by
DBI to perl scripts. The API is implemented by the DBI
Perl extension.
The 'Switch' is the code that 'dispatches' the DBI method
calls to the appropriate Driver for actual execution. The
Switch is also responsible for the dynamic loading of
Drivers, error checking/handling and other duties. The DBI
(database). Drivers contain implementations of the DBI
methods written using the private interface functions of
the corresponding Engine. Only authors of
sophisticated/multi-database applications or generic
library functions need be concerned with Drivers.
Notation and Conventions
DBI static 'top-level' class name
$dbh Database handle object
$sth Statement handle object
$drh Driver handle object (rarely seen or used in applications)
$h Any of the $??h handle types above
$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 by a function (if available, else -1)
$fh A filehandle
undef NULL values are represented by undefined values in perl
Note that Perl will automatically destroy database and
statement objects if all references to them are deleted.
Handle object attributes are shown as:
$h->{attribute_name} (type)
where type indicates the type of the value of the
attribute (if it's not a simple scalar):
\$ reference to a scalar: $h->{attr} or $a = ${$h->{attr}}
\@ reference to a list: $h->{attr}->[0] or @a = @{$h->{attr}}
\% reference to a hash: $h->{attr}->{a} or %a = %{$h->{attr}}
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 and 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. Be aware 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
native format of the corresponding Engine. Time Zone
effects are Engine/Driver dependent.
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.
Multiple SQL statements may not be combined in a single
statement handle, e.g., a single $sth.
Non-sequential record reads are not supported in this
version of the DBI. E.g., 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.
Character sets: Most databases which understand character
sets have a default global charset and text stored in the
database is, or should be, stored in that charset (if it's
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.
Naming Conventions
The DBI package and all packages below it (DBI::*) are
reserved for use by the DBI. Package names beginning with
DBD:: are reserved for use by DBI database drivers. All
environment variables used by the DBI or DBD's 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, SQL92 etc (portable)
MixedCase DBI API (portable), underscores are not used.
lower_case Driver or Engine specific (non-portable)
attributes. Private attribute names must be prefixed with
the driver name or suitable abbreviation (e.g., ora_type,
solid_charset etc).
Data Query Methods
The DBI allows an application to `prepare' a statement for
later execution. A prepared statement is identified by a
statement handle object, e.g., $sth.
Typical method call sequence for a select statement:
connect,
prepare,
execute, fetch, fetch, ... finish,
execute, fetch, fetch, ... finish,
execute, fetch, fetch, ... finish.
Typical method call sequence for a non-select statement:
connect,
prepare,
execute,
execute,
execute.
Placeholders and Bind Values
Some drivers support Placeholders and Bind Values. These
drivers allow a database statement to contain
placeholders, sometimes called parameter markers, that
indicate values 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 product_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. Undefined
values or undef can be used to indicate null values.
Without using placeholders, the insert statement above
would have to contain the literal values to be inserted
and it would have to be re-prepared and re-executed for
each row. With placeholders, the insert statement only
By avoiding the need to re-prepare the statement for each
row the application typically many times faster! Here's an
example:
my $sth = $dbh->prepare(q{
insert into sales (product_code, qty, price) values (?, ?, ?)
}) || die $dbh->errstr;
while(<>) {
chop;
my($product_code, $qty, $price) = split(/,/);
$sth->execute($product_code, $qty, $price) || die $dbh->errstr;
}
$dbh->commit || die $dbh->errstr;
See the execute and bind_param entries elsewhere in this
document for more details.
See the bind_column entry elsewhere in this document for a
related method used to associate perl variables with the
output columns of a select statement.
SQL - A Query Language
Most DBI drivers require applications to use a dialect of
SQL (the Structured Query Language) to interact with the
database engine. These links may provide some useful
information about SQL:
http://www.jcc.com/sql_stnd.html
http://w3.one.net/~jhoffman/sqltut.htm
http://skpc10.rdg.ac.uk/misc/sqltut.htm
http://epoch.CS.Berkeley.EDU:8000/sequoia/dba/montage/FAQ/SQL_TOC.html
http://www.bf.rmit.edu.au/Oracle/sql.html
The DBI itself does not mandate or require any particular
language to be used. It is language independant. In ODBC
terms it is always in pass-thru mode. The only requirement
is that queries and other statements must be expressed as
a single string of letters passed as the first argument to
the the prepare entry elsewhere in this document method.
THE DBI CLASS
DBI Class Methods
connect
$dbh = DBI->connect($data_source, $username, $password);
$dbh = DBI->connect($data_source, $username, $password, \%attr);
Establishes a database connection (session) to the
requested data_source. Returns a database handle
databases through multiple drivers can be made via the
DBI. Simply make one connect call for each and keep a
copy of each returned database handle.
The $data_source value should begin with
'dbi:driver_name:'. That prefix will be stripped off
and the driver_name part is used to specify the driver
(letter case is significant). As a convenience, if
the $data_source field is undefined or empty the DBI
will substitute the value of the environment variable
DBI_DSN if any.
If driver is not specified, the environment variable
DBI_DRIVER is used. If that variable is not set then
the connect dies.
Examples of $data_source values:
dbi:DriverName:database_name
dbi:DriverName:database_name@hostname
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. See
the documentation for the drivers you are using for a
description of the syntax it uses. Where a driver
needs to define it's own syntax for the data_source it
is recommended that they follow the ODBC style (the
last example above).
If $username or $password are undefined (rather than
empty) then the DBI will substitute the values of the
DBI_USER and DBI_PASS environment variables
respectively. The use of the environment for these
values is not recommended for security reasons. The
mechanism is only intended to simplify testing.
DBI->connect automatically installs the driver if it
has not been installed yet. Driver installation always
returns a valid driver handle or it dies with an error
message which 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, for which $DBI::errstr will hold
the error.
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
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 env vars if no data_source is
specified).
The AutoCommit and PrintError attributes for each
connection default to default to on (see the
AutoCommit and PrintError entries elsewhere in this
document for more information).
The \%attr parameter can be used to alter the default
settings of the PrintError, RaiseError and AutoCommit
attributes. For example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
These are currently the only defined uses for the
DBI->connect \%attr.
Portable applications should not assume that a single
driver will be able to support multiple simultaneous
sessions.
Where possible each session ($dbh) is independent from
the transactions in other sessions. This is useful
where you need to hold cursors open across
transactions, e.g., 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:' and, even if
it does, the embedded driver_name will be ignored. The
$dbh->{AutoCommit} attribute is undefined. The
$dbh->{PrintError} attribute is off. And the old
DBI_DBNAME env var is checked if DBI_DSN is not
defined.
available_drivers
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
for DBD::* modules through the directories in @INC. By
default a warning will be 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);
Returns a list of all data sources (databases)
available via the named driver. The driver will be
loaded if not already. If $driver is empty or undef
then the value of the DBI_DRIVER environment variable
will be used.
Note that many drivers have no way of knowing what
data sources might be available for it and thus,
typically, return an empty list.
trace
DBI->trace($trace_level)
DBI->trace($trace_level, $trace_file)
DBI trace information can be enabled for all handles
using this DBI class method. To enable trace
information for a specific handle use the similar
$h->trace method described elsewhere.
Use $trace_level 2 to see detailed call trace
information including parameters and return values.
The trace output is detailed and typically very
useful.
Use $trace_level 0 to disable the trace.
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.
See also the $h->trace() method and the DEBUGGING
entry elsewhere in this document for information about
the DBI_TRACE environment variable.
DBI Utility Functions
neat
$str = DBI::neat($value, $maxlen);
Strings will be quoted (but internal quotes will not
be escaped). Numeric values will usually 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 (0 or undef defaults to 400 characters) the
result string will be truncated to $maxlen-4 and ...'
will be appended.
This function is designed to format values for human
consumption. It is used internally by the DBI for the
trace entry elsewhere in this document output. It
should typically not be used for formating values for
database use.
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 ", ".
dump_results
$rows = DBI::dump_results($sth, $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 function is designed as a handy utility for
prototyping and testing queries. Since it uses the
neat_list entry elsewhere in this document which uses
the neat entry elsewhere in this document which
formats the string for reading by humans, it's not
recomended for data transfer applications.
DBI Dynamic Attributes
These attributes are always associated with the last
handle used.
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, because they
are associated with the last handle used, they should only
be used immediately after calling the method which 'sets'
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.
METHODS COMMON TO ALL HANDLES
err
$rv = $h->err;
Returns the native database engine error code from the
last driver function called.
errstr
$str = $h->errstr;
Returns the native database engine error message from
the last driver function called.
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 0 (false). If the driver does
not support SQLSTATE then state will return S1000
(General Error) for all errors.
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.
Use $trace_level 2 to see detailed call trace
information including parameters and return values.
The trace output is detailed and typically very
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.
See also the DBI->trace() method and the DEBUGGING
entry elsewhere in this document for information about
the DBI_TRACE environment variable.
func
$h->func(@func_arguments, $func_name);
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.
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.
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
... = $h->{AttributeName}; # get/read
Warn (boolean, inherited)
Enables useful warnings for certain bad practices.
Enabled by default. Some emulation layers, especially
those for perl4 interfaces, disable warnings.
CompatMode (boolean, inherited)
Used by emulation layers (such as Oraperl) to enable
application code.
InactiveDestroy (boolean)
This attribute can be used to disable the effect of
destroying a handle (which would normally close a
prepared statement or disconnect from the database
etc). It is specifically designed for use in unix
applications which 'fork' child processes. Either the
parent or the child process, but not both, should set
InactiveDestroy on all their handles.
PrintError (boolean, inherited)
This attribute can be used to force errors to generate
warnings (using warn) in addition to returning error
codes in the normal way. When set on, any method
which results in an error occuring ($DBI::err being
set true) will cause the DBI to effectively do
warn("$DBI::errstr"). Note that the contents of the
warning are currently just $DBI::errstr but that may
change and should not be relied upon.
By default DBI->connect sets PrintError on (except for
old-style connect usage, see connect for more
details).
If desired, the warnings can be caught and processed
using a $SIG{__WARN__} handler or modules like
CGI::ErrorWrap.
RaiseError (boolean, inherited)
This attribute can be used to force errors to raise
exceptions rather than simply return error codes in
the normal way. It defaults to off. When set on, any
method which results in an error occuring ($DBI::err
being set true) will cause the DBI to effectively do
croak("$DBI::errstr").
If PrintError is also on then the PrintError is done
before the RaiseError unless no __DIE__ handler has
been defined, in which case PrintError is skipped
since the croak will print the message.
Note that the contents of $@ are currently just
$DBI::errstr but that may change and should not be
relied upon.
ChopBlanks (boolean, inherited)
This attribute can be used to control the trimming of
trailing space characters from fixed width char
fields. No other field types are affected.
The default is false (it is possible that that may
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 must arrange to return undef
as the attribute value.
LongReadLen (integer, inherited)
This attribute may be used to control the maximum
length of 'long' (or 'blob') fields which the driver
will read from the database automatically when it
fetches each row of data. A value of 0 means don't
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. Most applications using long fields
will set this value to slightly larger than the
longest long field value which will be fetched.
Changing the value of LongReadLen for a statement
handle after it's been prepare()'d will typically have
no effect so it's usual to set LongReadLen on the $dbh
before calling prepare.
See the LongTruncOk entry elsewhere in this document
about truncation behaviour.
LongTruncOk (boolean, inherited)
This 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 fetching a
truncated long value will cause the fetch to fail.
(Applications should always take care 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.
DBI DATABASE HANDLE OBJECTS
Database Handle Methods
prepare
$sth = $dbh->prepare($statement) || die $dbh->errstr;
database engine and return a reference to a statement
handle object which can be used to get attributes of
the statement and invoke the the execute entry
elsewhere in this document method.
Note that prepare should never execute a statement,
even if it is not a select statement, it only prepares
it for execution. (Having said that, some drivers,
notably Oracle, will execute data definition
statements such as create/drop table when they are
prepared. In practice this is rarely a problem.)
Drivers for engines which don't have 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 likely to be
unable 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.
do
$rc = $dbh->do($statement) || die $dbh->errstr;
$rc = $dbh->do($statement, \%attr) || die $dbh->errstr;
$rv = $dbh->do($statement, \%attr, @bind_values) || ...
Prepare and execute a statement. Returns the number of
rows affected (-1 if not known or not available) or
undef on error.
This method is typically most useful for non-select
statements which either cannot be prepared in advance
(due to a limitation in the driver) or which do not
need to be executed repeatedly.
The default do method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows;
}
Example:
my $rows_deleted = $dbh->do(q{
delete from table
where status = 'DONE'
}) || die $dbh->errstr;
can be useful because it avoids the need to correctly
quote any variables in the $statement.
commit
$rc = $dbh->commit || die $dbh->errstr;
Commit (make permanent) the most recent series of
database changes if the database supports
transactions.
If the database supports transactions and AutoCommit
is on then the commit should issue a "commit
ineffective with AutoCommit" warning.
See also the Transactions entry elsewhere in this
document .
rollback
$rc = $dbh->rollback || die $dbh->errstr;
Roll-back (undo) the most recent series of uncommitted
database changes if the database supports
transactions.
If the database supports transactions and AutoCommit
is on then the rollback should issue a "rollback
ineffective with AutoCommit" warning.
See also the Transactions entry elsewhere in this
document .
disconnect
$rc = $dbh->disconnect || warn $dbh->errstr;
Disconnects the database from the database handle.
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
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
behaviour to ensure that incomplete transactions don't
get committed simply because Perl calls DESTROY on
every object before exiting.
If you disconnect from a database while you still have
active statement handles you will get a warning. The
statement handles should either be cleared (destroyed)
before disconnecting or the finish method called on
each one.
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.
The default implementation currently always returns
true without actually doing anything. Individual
drivers should implement this function in the most
suitable manner for their database engine.
Very few applications would have any use for this
method. See the specialist Apache::DBI module for one
example usage.
quote
$sql = $dbh->quote($string);
Quote a string literal for use 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\n");
For most database types quote would return 'Don''t'
(including the outer quotation marks).
An undefined $string value will be returned as NULL
(without quotation marks).
Quote may not be able to deal with all possible input
(such as binary data) and should not be relied upon
for security.
Database Handle Attributes
This section describes attributes specific to database
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 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 which need full transaction
behaviour can set $dbh->{AutoCommit}=0 (or via
connect) without having to check the value was
assigned okay.
For the purposes of this description we can divide
databases into three categories:
Database which don't support transactions at all.
Database in which a transaction is always active.
Database in which a transaction must be explicitly started ('BEGIN WORK').
* Database 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.
* Database in which a transaction is always active
These are typically mainstream commercial relational
databases with 'ANSI standandard' transaction
behaviour.
If AutoCommit is off then changes to the database
won't have any lasting effect unless the commit entry
rollback entry elsewhere in this document 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 were to have called commit automatically after
every successful database operation. In other words,
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 should issue a the
commit entry elsewhere in this document in most
drivers.
Changing AutoCommit from on to off should have no
immediate effect.
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.
* Database in which a transaction must be explicitly
started
For these database the intention is to have them act
like databases in which a transaction is always active
(as described above).
To do this the DBI driver will automatically begin a
transaction when AutoCommit is turned off (from the
default on state) and will automatically begin another
transaction after a the commit entry elsewhere in this
document or the rollback entry elsewhere in this
document .
In this way, the application does not have to treat
these databases as a special case.
DBI STATEMENT HANDLE OBJECTS
Statement Handle Methods
bind_param
$rc = $sth->bind_param($param_num, $bind_value) || die $sth->errstr;
$rv = $sth->bind_param($param_num, $bind_value, \%attr) || ...
$rv = $sth->bind_param($param_num, $bind_value, $bind_type) || ...
(assign/associate) 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 :1, :2 etc and :name style
placeholders in addition to ? but their use is not
portable.
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 validating the statement and creating a query
execution plan for it. For example:
"select name, age from ?" # wrong
"select name, ? from people" # wrong
Also, placeholders can only represent single scalar
values, so this statement, for example, won't work as
expected for more than one value:
"select name, age from people where name in (?)" # wrong
The \%attr parameter can be used to specify 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 inplace of the attr hash reference.
This example is equivalent to the one above:
$sth->bind_param(1, $value, SQL_INTEGER);
The TYPE cannot be changed after the first bind_param
call (but it can be left unspecified, in which case it
defaults to the previous value).
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
values.
bind_param_inout
$rc = $sth->bind_param_inout($param_num, \$bind_value, $max_len) || die $sth->errstr;
$rv = $sth->bind_param_inout($param_num, \$bind_value, $max_len, \%attr) || ...
$rv = $sth->bind_param_inout($param_num, \$bind_value, $max_len, $bind_type) || ...
This method acts like the bind_param entry elsewhere
in this document but also enables values to be output
from (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.
The additional $max_len parameter specifies the amount
of memory to allocate to $bind_value for the new
value. Truncation behaviour, if the value is longer
than $max_len, is currently undefined.
It is expected that few drivers will support this
method. The only driver currently known to do so is
DBD::Oracle. It should not be used for database
independent applications.
execute
$rv = $sth->execute || die $sth->errstr;
$rv = $sth->execute(@bind_values) || 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
(see below). It is always important to check the
return status of execute (and most other DBI methods).
For a non-select statement execute returns the number
of rows affected (if known). Zero rows is returned as
"0E0" which Perl will treat as 0 but will regard as
true. If the number of rows affected is not known then
execute returns -1.
For select statements execute simply 'starts' the
query within the Engine. Use one of the fetch methods
to retreive the data after calling execute. The
execute method does not return the number of rows that
will be returned by the query (because most Engines
can't tell in advance), it simply returns a true
value.
If any arguments are given then execute will
effectively call the bind_param entry elsewhere in
treated as SQL_VARCHAR types unless the driver can
determine the correct type (which is rare) or
bind_param (or bind_param_inout) has already been used
to specify the type.
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. If there are no
more rows fetchrow_arrayref returns undef. Null
values are returned as undef. This is the fastest way
to fetch data, particularly if used with
$sth->bind_columns.
fetchrow_array
@ary = $sth->fetchrow_array;
An alternative to fetchrow_arrayref. Fetches the next
row of data and returns it as an array holding the
field values. If there are no more rows fetchrow_array
returns an empty list. Null values are returned as
undef.
fetchrow_hashref
$hash_ref = $sth->fetchrow_hashref;
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
values are returned as undef. If there are no more
rows fetchhash returns undef.
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.
Because of the extra work fetchrow_hashref and perl
have to perform it is not as efficient as
fetchrow_arrayref or fetchrow_array and is not
recommended where performance is very important.
Currently a new hash reference is returned for each
row. This is likely to change in the future so don't
rely on it.
fetchall_arrayref
$tbl_ary_ref = $sth->fetchall_arrayref;
the data to be returned from a prepared statement. It
returns a reference to an array which contains one
array reference per row (as returned by
fetchrow_arrayref).
If there are no rows to return, fetchall_arrayref
returns a reference to an empty array.
finish
$rc = $sth->finish;
Indicates that no more data will be fetched from this
statement before it is either prepared again or
destroyed. It can sometimes be helpful to call this
method where appropriate in order to allow the server
to free up any internal resources (such as read locks)
currently being held. It does not affect the
transaction status of the session.
The finish method has nothing to do with transactions.
It's mostly an internal 'housekeeping' method. There's
no need to call finish if you're about to destroy or
re-use the statement handle. See also the disconnect
entry elsewhere in this document .
rows
$rv = $sth->rows;
Returns the number of rows affected by the last
database altering command, or -1 if not known or not
available.
Generally you can only rely on a row count after a do
or 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.
bind_col
$rc = $sth->bind_col($column_number, \$var_to_bind);
$rc = $sth->bind_col($column_number, \$var_to_bind, \%attr);
Binds a column (field) of a select statement to a perl
variable. Whenever a row is fetched from the database
values manually. This makes using bound variables very
efficient. See bind_columns below for an example.
Note that column numbers count up from 1.
The binding is performed at a very low level using
perl aliasing so there is no extra copying taking
place. So long as the driver uses the correct internal
DBI call to get the array the fetch function returns,
it will automatically support column binding.
The the bind_param entry elsewhere in this document
method performs a similar function for input
variables. See the section on /"Placeholders and Bind
Values for more information.
bind_columns
$rc = $sth->bind_columns(\%attr, @list_of_refs_to_vars_to_bind);
e.g.
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ select region, sales from sales_by_region });
my($region, $sales);
# Bind perl variables to columns.
$rv = $sth->bind_columns(undef, \$region, \$sales);
# you can also use perl's \(...) syntax (see perlref docs):
# $sth->bind_columns(undef, \($region, $sales));
# Column binding is the most efficient way to fetch data
$sth->execute;
while($sth->fetch) {
print "$region: $sales\n";
}
Calls bind_col for each column of the select
statement. bind_columns will croak if the number of
references does not match the number of fields.
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:
some or all of these attributes until after $sth->execute
has been called.
NUM_OF_FIELDS (integer, read-only)
Number of fields (columns) the prepared statement will
return. Non-select statements will have NUM_OF_FIELDS
== 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.
print "First column name: $sth->{NAME}->[0]\n";
NULLABLE (array-ref, read-only)
Returns a reference to an array indicating the
possibility of each column returning a null.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
CursorName (string, read-only)
Returns the name of the cursor associated with the
statement handle if available. If not available or the
database driver does not support the "where current of
..." SQL syntax then it returns undef.
TRANSACTIONS
Transactions are a fundamental part of any robust database
system. They protect against errors and database
corruption by ensuring that sets of related changes to the
database take place in atomic (indivisible, all-or-
nothing) units.
See the AutoCommit entry elsewhere in this document for
details of using AutoCommit with various types of
database.
Robust Applications
This section applies to databases which support
transactions and where AutoCommit is off.
The recommended way to implement robust transactions in
Perl applications is to make use of eval { ... } (which is
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
};
if ($@) {
$dbh->rollback;
# add other application on-error-clean-up code here
}
else {
$dbh->commit;
}
The code in foo(), or any other code executed from within
the curly braces, can be implemented in this way:
$h->method(@args) || die $h->errstr
or the $h->{RaiseError} attribute can be set on, in which
case the DBI will automatically croak() on error so you
don't have to test the return value of each method call.
See the RaiseError entry elsewhere in this document for
more details.
A major advantage of the eval approach is that the
transaction will be properly rolled back if any code in
the inner application croaks or 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 recommended.
HANDLING BLOB / LONG 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 for such fields. The $h->{LongTruncOk} attribute
is used to determine how to behave if a fetched value
can't fit into the buffer.
SIMPLE EXAMPLE
my $dbh = DBI->connect("dbi:Oracle:$data_source", $user, $password)
|| die "Can't connect to $data_source: $DBI::errstr";
SELECT name, phone
FROM mytelbook
}) || die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
|| die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "$sth->{NAME}->[0]: $sth->{NAME}->[1]\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
warn $DBI::errstr if $DBI::err;
$sth->finish;
DEBUGGING
In addition to the the trace entry elsewhere in this
document 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 for a single command:
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 will be appended to that file.
See also the the trace entry elsewhere in this document
method.
WARNINGS
The DBI is alpha software. It is only 'alpha' because the
interface (api) is not finalised. The alpha status does
not reflect code quality.
SEE ALSO
Database Documentation
SQL Language Reference Manual.
Books and Journals
Programming Perl 2nd Ed. by Larry Wall, Tom Christiansen & Randal Schwartz.
Learning Perl by Randal Schwartz.
The Perl Journal, April 1997.
Manual Pages
the perl(1) manpage, the perlmod(1) manpage, the
perlbook(1) manpage
Mailing List
The dbi-users mailing list is the primary means of
communication among uses of the DBI and its related
modules. Subscribe and unsubscribe via:
http://www.fugue.com/dbi
Mailing list archives are held at:
http://www.rosat.mpe-garching.mpg.de/mailing-lists/PerlDB-Interest/
http://outside.organic.com/mail-archives/dbi-users/
http://www.coe.missouri.edu/~faq/lists/dbi.html
Assorted Related WWW Links
The DBI 'Home Page' (not maintained by me):
http://www.arcana.co.uk/technologia/DBI
Other related links:
http://www-ccs.cs.umass.edu/db.html
http://www.odmg.org/odmg93/updates_dbarry.html
http://www.jcc.com/sql_stnd.html
ftp://alpha.gnu.ai.mit.edu/gnu/gnusql-0.7b3.tar.gz
FAQ
Please also read the DBI FAQ which is installed as a
DBI::FAQ module so 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) 1995,1996,1997 Tim Bunce.
England. All rights reserved.
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.
SUPPORT / WARRANTY
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY
KIND.
Commercial support agreements for Perl and the DBI,
DBD::Oracle and Oraperl modules can be arranged via The
Perl Clinic. See http://www.perl.co.uk/tpc for more
details.
OUTSTANDING ISSUES TO DO
data types (ISO type numbers and type name conversions)
error handling
data dictionary methods
test harness support methods
portability
blob_read
etc
FREQUENTLY ASKED QUESTIONS
See the DBI FAQ for a more comprehensive list of FAQs. Use
the perldoc DBI::FAQ command to read it.
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/perl/faq/idiots-guide.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www.perl.com/perl/faq/perl-cgi-faq.html
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:
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/
A driver build fails because it can't find DBIXS.h
The installed location of the DBIXS.h file changed with
0.77 (it was being installed into the 'wrong' directory
but that's where driver developers came to expect it to
be). The first thing to do is check to see if you have the
latest version of your driver. Driver authors will be
releasing new versions which use the new location. If you
have the latest then ask for a new release. You can edit
the Makefile.PL file yourself. Change the part which reads
"-I.../DBI" so it reads "-I.../auto/DBI" (where ... is a
string of non-space characters).
Has the DBI and DBD::Foo been ported to NT / Win32?
The latest version of the DBI and, at least, the
DBD::Oracle module will build - without changes - on
NT/Win32 if your are using the standard Perl 5.004 and not
the ActiveWare port.
Jeffrey Urlwin <jurlwin@access.digex.net> (or
<jurlwin@hq.caci.com>) is helping me with the port
(actually he's doing it and I'm integrating the changes
:-).
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 writtem well.
See also the "Does Perl have a year 2000 problem?" section
KNOWN DRIVER MODULES
ODBC - DBD::ODBC
Author: Tim Bunce
Email: dbi-users@fugue.com
Oracle - DBD::Oracle
Author: Tim Bunce
Email: dbi-users@fugue.com
Ingres - DBD::Ingres
Author: Henrik Tougaard
Email: ht@datani.dk, dbi-users@fugue.com
mSQL - DBD::mSQL
DB2 - DBD::DB2
Empress - DBD::Empress
Informix - DBD::Informix
Author: Jonathan Leffler
Email: johnl@informix.com, dbi-users@fugue.com
Solid - DBD::Solid
Author: Thomas Wenrich
Email: wenrich@site58.ping.at, dbi-users@fugue.com
Postgres - DBD::Pg
Author: Edmund Mergl
Email: E.Mergl@bawue.de, dbi-users@fugue.com
Fulcrum SearchServer - DBD::Fulcrum
Author: Davide Migliavacca
Email: davide.migliavacca@inferentia.it
OTHER RELATED WORK AND PERL MODULES
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
Carl Declerck <carl@miskatonic.inbe.net>
Terry Greenlaw <z50816@mip.mar.lmco.com>
Carl is developing a generic proxy object module which
could form the basis of a DBD::Proxy driver in the
future. Terry is doing something similar.
SQL Parser
Hugo van der Sanden <hv@crypt.compulink.co.uk>
Stephen Zander <stephen.zander@mckesson.com>
Based on the O'Reilly lex/yacc book examples and
byacc.