为什么d.product无法获得p.product的价值？

Question

mysql_query("SELECT  d.detail_id, p.product_name,p.product_price, d.quantity, d.subtotal, p.product_id
    FROM    order_detail AS d, product_list AS p, order_tbl AS r
    WHERE   d.order_id = r.or_id
    AND     d.product_id = p.product_id //---this is the problem
    AND     r.or_id = $result[or_id]
    ") or die(mysql_error());