$sql = "SELECT $field
FROM ".TB_PREF."sales_order_details d
WHERE order_no =
- (SELECT order_no FROM 1_sales_orders o
+ (SELECT order_no FROM ".TB_PREF."sales_orders o
WHERE debtor_no=" . db_escape($order->customer_id) . "
ORDER BY order_no DESC LIMIT 1)
ORDER BY d.id DESC LIMIT 1";