Opencart database query not working. Why?

Question

opencart

Need help from Opencarta gurus :) There is a function, the code is below. Can you please tell me why it can not be executed, or an empty result can be returned? The function sometimes produces a result, but it is still not correct. But basically the result is empty. Or tell me how you can see the already formed request that is sent to the database?

protected function getTotalBought($product_id, $order_status_id = 0){
          $bought = 0;
          $filter_status = " AND o.order_status_id > 0";
          if($order_status_id ) {
            $filter_status = " AND o.order_status_id IN (".implode(",",$order_status_id).")";
          }
          $query = $this->db->query("SELECT sum(quantity) as `total` FROM `" . DB_PREFIX . "order_product` op
            LEFT JOIN `".DB_PREFIX."order` AS o ON op.order_id = o.order_id WHERE op.product_id = ".$product_id.$filter_status);

          if($query->num_rows){
            return $query->row['total'];
          }
          return 0;
        }

Answer 1

You can see the request that has been formed in the standard way, for example like this
Next, you can check the received request through phpmyadmin and find errors