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:
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 Skip Informix 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.
Anything else based on DBI can be easily added by locating the
$dbh. Patches or suggestions welcome.
Other options are described in SQL::Abstract.
SQL::Abstract::select, but accepts additional
$order parameter is required if
$rows is specified.
$fields parameter is required, but can be set to
'*' (all these get set to
$where parameter is also required. It can be a hashref or an arrayref, or
SQL::Abstract::where, but accepts additional
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.
croak if you try to use it for other syntaxes.
$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
Returns a regular
WHERE clause if no limits are set.
See SQL::Abstract for these methods.
delete are not provided with any
LIMIT emulation in this release, and no support is planned at the moment. But patches would be welcome.
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.
SELECT foo, bar FROM my_table WHERE some_conditions
$sql with the leading
SELECT keyword removed.
order_cols_up represents the sort column(s) and direction(s) specified in the
order_cols_down represents the opposite sort.
$last = $rows + $offset
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.
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
$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.
BETWEEN $offset AND $last clause is replaced with
< $rows if <$offset == 0>.
Sybase Anything not otherwise known to this module.
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.
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
where instead of
order parameter passed to the
where call. You can get an
ORDER BY clause from this by calling
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.
$dialect parameter that can be passed to the constructor or to the
where methods can be a number of things. The module will attempt to determine the appropriate syntax to use.
$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.
Thanks to Paul Falbe for the Informix implementation.
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.
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.
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.