set sql variable before main query

Is it possible to set sql variable before other queries?(not $this->db->query())

I'd like to do something like this.

PHP Code:

$this->db->query("SET @min_id := (SELECT min_id FROM shop_parameter)");
$this->db->select('item_id');
$this->db->from('products');
$this->db->where('(item_id > @min_id',NULL,FALSE);
$this->db->where(array('item_stat >'=>10));
$query=$this->db->get();
foreach($query->result() as $r){
   $products[$r->item_id]=$r; }
}