Mysql user defined variable

[eluser]dannythebestguy[/eluser]
I want to use a user defined Mysql variable in a Select statement and run it through query function.

$sql = 'Select @var := col1 - col2 FROM table1 WHERE col3 = @var';
$query = $this->db->query($sql);
$data = $query->result_array();

But CI is not returning the records.

Please let me know how to go about and to set the variable too.

Thnx

Dinesh