NAME

       DBIx::Class::Storage::DBI::MSSQL - Base Class for Microsoft SQL Server support in
       DBIx::Class

SYNOPSIS

       This is the base class for Microsoft SQL Server support, used by
       DBIx::Class::Storage::DBI::ODBC::Microsoft_SQL_Server and
       DBIx::Class::Storage::DBI::Sybase::Microsoft_SQL_Server.

IMPLEMENTATION NOTES

   IDENTITY information
       Microsoft SQL Server supports three methods of retrieving the IDENTITY value for inserted
       row: IDENT_CURRENT, @@IDENTITY, and SCOPE_IDENTITY().  SCOPE_IDENTITY is used here because
       it is the safest.  However, it must be called is the same execute statement, not just the
       same connection.

       So, this implementation appends a SELECT SCOPE_IDENTITY() statement onto each INSERT to
       accommodate that requirement.

       "SELECT @@IDENTITY" can also be used by issuing:

         $self->_identity_method('@@identity');

       it will only be used if SCOPE_IDENTITY() fails.

       This is more dangerous, as inserting into a table with an on insert trigger that inserts
       into another table with an identity will give erroneous results on recent versions of SQL
       Server.

   identity insert
       Be aware that we have tried to make things as simple as possible for our users.  For MSSQL
       that means that when a user tries to create a row, while supplying an explicit value for
       an autoincrementing column, we will try to issue the appropriate database call to make
       this possible, namely "SET IDENTITY_INSERT $table_name ON". Unfortunately this operation
       in MSSQL requires the "db_ddladmin" privilege, which is normally not included in the
       standard write-permissions.

   Ordered Subselects
       If you attempted the following query (among many others) in Microsoft SQL Server

        $rs->search ({}, {
         prefetch => 'relation',
         rows => 2,
         offset => 3,
        });

       You may be surprised to receive an exception. The reason for this is a quirk in the MSSQL
       engine itself, and sadly doesn't have a sensible workaround due to the way DBIC is built.
       DBIC can do truly wonderful things with the aid of subselects, and does so automatically
       when necessary. The list of situations when a subselect is necessary is long and still
       changes often, so it can not be exhaustively enumerated here. The general rule of thumb is
       a joined has_many relationship with limit/group applied to the left part of the join.

       In its "pursuit of standards" Microsft SQL Server goes to great lengths to forbid the use
       of ordered subselects. This breaks a very useful group of searches like "Give me things
       number 4 to 6 (ordered by name), and prefetch all their relations, no matter how many".
       While there is a hack which fools the syntax checker, the optimizer may still elect to
       break the subselect.  Testing has determined that while such breakage does occur (the test
       suite contains an explicit test which demonstrates the problem), it is relative rare. The
       benefits of ordered subselects are on the other hand too great to be outright disabled for
       MSSQL.

       Thus compromise between usability and perfection is the MSSQL-specific resultset attribute
       "unsafe_subselect_ok".  It is deliberately not possible to set this on the Storage level,
       as the user should inspect (and preferably regression-test) the return of every such
       ResultSet individually. The example above would work if written like:

        $rs->search ({}, {
         unsafe_subselect_ok => 1,
         prefetch => 'relation',
         rows => 2,
         offset => 3,
        });

       If it is possible to rewrite the search() in a way that will avoid the need for this flag
       - you are urged to do so. If DBIC internals insist that an ordered subselect is necessary
       for an operation, and you believe there is a different/better way to get the same result -
       please file a bugreport.

FURTHER QUESTIONS?

       Check the list of additional DBIC resources.

COPYRIGHT AND LICENSE

       This module is free software copyright by the DBIx::Class (DBIC) authors. You can
       redistribute it and/or modify it under the same terms as the DBIx::Class library.