SQL::Abstract::Limit - portable LIMIT emulation
use SQL::Abstract::Limit; my $sql = SQL::Abstract::Limit->new( limit_dialect => 'LimitOffset' );; # or autodetect from a DBI $dbh: my $sql = SQL::Abstract::Limit->new( limit_dialect => $dbh ); # or from a Class::DBI class: my $sql = SQL::Abstract::Limit->new( limit_dialect => 'My::CDBI::App' ); # or object: my $obj = My::CDBI::App->retrieve( $id ); my $sql = SQL::Abstract::Limit->new( limit_dialect => $obj ); # generate SQL: my ( $stmt, @bind ) = $sql->select( $table, \@fields, \%where, \@order, $limit, $offset ); # Then, use these in your DBI statements my $sth = $dbh->prepare( $stmt ); $sth->execute( @bind ); # Just generate the WHERE clause (only available for some syntaxes) my ( $stmt, @bind ) = $sql->where( \%where, \@order, $limit, $offset );
Portability layer for LIMIT emulation.
All settings are optional.
Sets the default syntax model to use for emulating a LIMIT $rows OFFSET $offset clause. Default setting is GenericSubQ. You can still pass other syntax settings in method calls, this just sets the default. Possible values are:
LIMIT $rows OFFSET $offset
GenericSubQ
LimitOffset PostgreSQL, SQLite LimitXY MySQL, MaxDB, anything that uses SQL::Statement LimitYX SQLite (optional) RowsTo InterBase/FireBird Top SQL/Server, MS Access RowNum Oracle FetchFirst DB2 First Informix # not implemented yet GenericSubQ Sybase, plus any databases not recognised by this module $dbh a DBI database handle CDBI subclass CDBI object other DBI-based thing
The first group are implemented by appending a short clause to the end of the statement. The second group require more intricate wrapping of the original statement in subselects.
You can pass a DBI database handle, and the module will figure out which dialect to use.
You can pass a Class::DBI subclass or object, and the module will find the $dbh and use it to find the dialect.
$dbh
Anything else based on DBI can be easily added by locating the $dbh. Patches or suggestions welcome.
Other options are described in SQL::Abstract.
Same as SQL::Abstract::select, but accepts additional $rows, $offset and $dialect parameters.
SQL::Abstract::select
$rows
$offset
$dialect
The $order parameter is required if $rows is specified.
$order
The $fields parameter is required, but can be set to undef, '' or '*' (all these get set to '*').
$fields
undef
''
'*'
The $where parameter is also required. It can be a hashref or an arrayref, or undef.
$where
Same as SQL::Abstract::where, but accepts additional $rows, $offset and $dialect parameters.
SQL::Abstract::where
Some SQL dialects support syntaxes that can be applied as simple phrases tacked on to the end of the WHERE clause. These are:
LimitOffset LimitXY LimitYX RowsTo
This method returns a modified WHERE clause, if the limit syntax is set to one of these options (either in the call to where or in the constructor), and if $rows is passed in.
where
Dies via croak if you try to use it for other syntaxes.
croak
$order is required if $rows is set.
$where is required if any other parameters are specified. It can be a hashref or an arrayref, or undef.
Returns a regular WHERE clause if no limits are set.
WHERE
See SQL::Abstract for these methods.
update and delete are not provided with any LIMIT emulation in this release, and no support is planned at the moment. But patches would be welcome.
update
delete
LIMIT
The following dialects are available for emulating the LIMIT clause. In each case, $sql represents the SQL statement generated by SQL::Abstract::select, minus the ORDER BY clause, e.g.
$sql
SELECT foo, bar FROM my_table WHERE some_conditions
$sql_after_select represents $sql with the leading SELECT keyword removed.
$sql_after_select
SELECT
order_cols_up represents the sort column(s) and direction(s) specified in the order parameter.
order_cols_up
order
order_cols_down represents the opposite sort.
order_cols_down
$last = $rows + $offset
$sql ORDER BY order_cols_up LIMIT $rows OFFSET $offset
or
$sql ORDER BY order_cols_up LIMIT $rows
if $offset == 0.
$offset == 0
PostgreSQL SQLite
$sql ORDER BY order_cols_up LIMIT $offset, $rows
MySQL
$sql ORDER BY order_cols_up LIMIT $rows, $offset
SQLite understands this syntax, or LimitOffset. If autodetecting the dialect, it will be set to LimitOffset.
$sql ORDER BY order_cols_up ROWS $offset TO $last
InterBase FireBird
SELECT * FROM ( SELECT TOP $rows * FROM ( SELECT TOP $last $sql_after_select ORDER BY order_cols_up ) AS foo ORDER BY order_cols_down ) AS bar ORDER BY order_cols_up
SQL/Server MS Access
Oracle numbers rows from 1, not zero, so here $offset has been incremented by 1.
SELECT * FROM ( SELECT A.*, ROWNUM r FROM ( $sql ORDER BY order_cols_up ) A WHERE ROWNUM <= $last ) B WHERE r >= $offset
Oracle
SELECT * FROM ( SELECT * FROM ( $sql ORDER BY order_cols_up FETCH FIRST $last ROWS ONLY ) foo ORDER BY order_cols_down FETCH FIRST $rows ROWS ONLY ) bar ORDER BY order_cols_up
IBM DB2
When all else fails, this should work for many databases, but it is probably fairly slow.
This method relies on having a column with unique values as the first column in the SELECT clause (i.e. the first column in the \@fields parameter). The results will be sorted by that unique column, so any $order parameter is ignored, unless it matches the unique column, in which case the direction of the sort is honoured.
\@fields
SELECT field_list FROM $table X WHERE where_clause AND ( SELECT COUNT(*) FROM $table WHERE $pk > X.$pk ) BETWEEN $offset AND $last ORDER BY $pk $asc_desc
$pk is the first column in field_list.
$pk
field_list
$asc_desc is the opposite direction to that specified in the method call. So if you want the final results sorted ASC, say so, and it gets flipped internally, but the results come out as you'd expect. I think.
$asc_desc
ASC
The BETWEEN $offset AND $last clause is replaced with < $rows if <$offset == 0>.
BETWEEN $offset AND $last
< $rows
Sybase Anything not otherwise known to this module.
Looks to be identical to Top, e.g. SELECT FIRST 10 * FROM table. Can probably be implemented in a very similar way, but not done yet.
Top
SELECT FIRST 10 * FROM table
Informix
You can create your own syntax by making a subclass that provides an emulate_limit method. This might be useful if you are using stored procedures to provide more efficient paging.
emulate_limit
This is the SQL statement built by SQL::Abstract, but without the ORDER BY clause, e.g.
SELECT foo, bar FROM my_table WHERE conditions
or just
WHERE conditions
if calling where instead of select.
select
The order parameter passed to the select or where call. You can get an ORDER BY clause from this by calling
ORDER BY
my $order_by = $self->_order_by( $order );
You can get a pair of ORDER BY clauses that sort in opposite directions by saying
my ( $up, $down ) = $self->_order_directions( $order );
The method should return a suitably modified SQL statement.
The $dialect parameter that can be passed to the constructor or to the select and where methods can be a number of things. The module will attempt to determine the appropriate syntax to use.
Supported $dialect things are:
dialect name (e.g. LimitOffset, RowsTo, Top etc.) database moniker (e.g. Oracle, SQLite etc.) DBI database handle Class::DBI subclass or object
Paging results sets is a complicated undertaking, with several competing factors to take into account. This module does not magically give you the optimum paging solution for your situation. It gives you a solution that may be good enough in many situations. But if your tables are large, the SQL generated here will often not be efficient. Or if your queries involve joins or other complications, you will probably need to look elsewhere.
But if your tables aren't too huge, and your queries straightforward, you can just plug this module in and move on to your next task.
Thanks to Aaron Johnson for the Top syntax model (SQL/Server and MS Access).
Thanks to Emanuele Zeppieri for the IBM DB2 syntax model.
Find more syntaxes to implement.
Test the syntaxes against real databases. I only have access to MySQL. Reports of success or failure would be great.
SQL::Abstract, DBI::Const::GetInfoType, Carp.
DBIx::SQLEngine, DBIx::SearchBuilder, DBIx::RecordSet.
Please report all bugs via the CPAN Request Tracker at http://rt.cpan.org/NoAuth/Bugs.html?Dist=SQL-Abstract-Limit.
Copyright 2004 by David Baird.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
David Baird, cpan@riverside-cms.co.uk
cpan@riverside-cms.co.uk
A few CPAN modules do this for a few databases, but the most comprehensive seem to be DBIx::SQLEngine, DBIx::SearchBuilder and DBIx::RecordSet.
Have a look in the source code for my notes on how these modules tackle similar problems.
To install SQL::Abstract::Limit, copy and paste the appropriate command in to your terminal.
cpanm
cpanm SQL::Abstract::Limit
CPAN shell
perl -MCPAN -e shell install SQL::Abstract::Limit
For more information on module installation, please visit the detailed CPAN module installation guide.