set sql variable before main query

Ok, I've used two successive $this->db->query()

Code:

$randomitem=array();
$valquery = "SELECT @rid := FLOOR(1+RAND()*item_id) FROM products ORDER BY item_id DESC LIMIT 1";
$this->db->query($valquery);
$mainquery  = "SELECT item_id FROM products WHERE item_id < @rid AND item_stat > 10 ORDER BY item_id DESC LIMIT 2";
$query = $this->db->query($mainquery);
foreach($query->result() as $r){ $randomitem[]=$r->item_id; }