Provided by: libsql-abstract-limit-perl_0.14.3-1_all 
NAME
SQL::Abstract::Limit - portable LIMIT emulation
SYNOPSIS
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 );
DESCRIPTION
Portability layer for LIMIT emulation.
new( case => 'lower', cmp => 'like', logic => 'and', convert => 'upper', limit_dialect =>
'Top' )
All settings are optional.
limit_dialect
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.
select( $table, \@fields, $where, [ \@order, [ $rows, [ $offset ], [ $dialect ] ] ] )
Same as "SQL::Abstract::select", but accepts additional $rows, $offset and $dialect
parameters.
The $order parameter is required if $rows is specified.
The $fields parameter is required, but can be set to "undef", '' or '*' (all these get
set to '*').
The $where parameter is also required. It can be a hashref or an arrayref, or "undef".
where( [ $where, [ \@order, [ $rows, [ $offset ], [ $dialect ] ] ] ] )
Same as "SQL::Abstract::where", but accepts additional $rows, $offset and $dialect
parameters.
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.
Dies via "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 "undef".
Returns a regular "WHERE" clause if no limits are set.
insert
update
delete
values
generate
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.
Limit emulation
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_after_select represents $sql with the leading "SELECT" keyword removed.
"order_cols_up" represents the sort column(s) and direction(s) specified in the "order"
parameter.
"order_cols_down" represents the opposite sort.
"$last = $rows + $offset"
LimitOffset
Syntax
$sql ORDER BY order_cols_up LIMIT $rows OFFSET $offset
or
$sql ORDER BY order_cols_up LIMIT $rows
if "$offset == 0".
Databases
PostgreSQL
SQLite
LimitXY
Syntax
$sql ORDER BY order_cols_up LIMIT $offset, $rows
or
$sql ORDER BY order_cols_up LIMIT $rows
if "$offset == 0".
Databases
MySQL
LimitYX
Syntax
$sql ORDER BY order_cols_up LIMIT $rows, $offset
or
$sql ORDER BY order_cols_up LIMIT $rows
if "$offset == 0".
Databases
SQLite understands this syntax, or LimitOffset. If autodetecting the
dialect, it will be set to LimitOffset.
RowsTo
Syntax
$sql ORDER BY order_cols_up ROWS $offset TO $last
Databases
InterBase
FireBird
Top
Syntax
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
Databases
SQL/Server
MS Access
RowNum
Syntax 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
Databases
Oracle
FetchFirst
Syntax
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
Databases
IBM DB2
GenericSubQ
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 honored.
Syntax
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".
$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.
The "BETWEEN $offset AND $last" clause is replaced with "< $rows" if <$offset
== 0>.
Databases
Sybase Anything not otherwise known to this module.
Skip
Syntax
select skip 5 limit 5 * from customer
which will take rows 6 through 10 in the select.
Databases
Informix
SUBCLASSING
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( $self, $sql, $order, $rows, $offset )
$sql
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".
$order
The "order" parameter passed to the "select" or "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.
AUTO-DETECTING THE DIALECT
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
CAVEATS
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.
ACKNOWLEDGEMENTS
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.
TODO
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.
DEPENDENCIES
SQL::Abstract, DBI::Const::GetInfoType, Carp.
SEE ALSO
DBIx::SQLEngine, DBIx::SearchBuilder, DBIx::RecordSet.
BUGS
Please report all bugs (patches welcome) via GitHub at
<https://github.com/asb-capfan/SQL-Abstract-Limit> or via the CPAN Request Tracker at
<http://rt.cpan.org/NoAuth/Bugs.html?Dist=SQL-Abstract-Limit>.
AUTHOR, COPYRIGHT AND LICENSE
Copyright 2004-2020 by David Baird. Currently maintained by Alexander Becker.
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
HOW IS IT DONE ELSEWHERE
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.