OrderBy Issue

$sub1 = $this->subquery->start_union();
	$sub1->select('keyword','link','frequency')
		->from('link_result')
		->limit($limit, $offset)
	     	->order_by($sort_by, $sort_order);
$sub2 = $this->subquery->start_union();
	$sub2->select('keyword','link','frequency')
		->from('sub_link_result')
		->limit($limit, $offset)
		->order_by($sort_by, $sort_order);
$this->subquery->end_union();
$q = $this->db->get();