yii optgroups

public function getGroupedCategories() { 
      $connection = Yii::app()->db;
      $sql='SELECT c.id, c.name, (Select name from linkcategory WHERE id = c.parent) AS `group` FROM linkcategory c WHERE c.parent > 0 ORDER BY c.parent, c.name';

      $t = $connection->createCommand($sql)->queryAll();            
      return $t;
} 

dropDownList($model, 'category', CHtml::listData(
         $model->getGroupedCategories(), 'id', 'name', 'group')
         ); 
?>