WHERE句でスカラサブクエリを使う

 my $rs   = $self->schema->resultset('Category');
 my $subq = sprintf
   " = (SELECT parent_category_id FROM category WHERE category_id = %d)",
   $category_id;
 $rs = $rs->search( { parent_category_id => \$subq } );
 $rs = $rs->search_literal( 'category_id <> parent_category_id', { order_by => 'category_id' } );