Codeigniter多表查询sum，group_by

Question

这个查询想要求帮助，如果有人可以，或者是关于如何写出来的建议，谢谢

public function current_user()
{
    //összes számla
    $result = $this->db->select('u.*, sum(a.advance) as advance, sum(p.price) as price')
                       ->from('advance a, products p, users u')
                       ->where('a.user_id = u.id')
                       ->where('p.user_id = u.id')
                       ->group_by('u.name','desc')
                       ->get();
    if($result->num_rows() > 0){
        return $result->result();        
    } else {
        return false; 
    }
}