MySQL-多個2列，在同一表中更新

Question

我想通過將quantity列乘以cost列來更新表tbl_orders的total列。

UPDATE `tbl_orders` SET 
   total = (SELECT quantity * cost AS n1 FROM `tbl_orders` WHERE orderid = 167 LIMIT 1) 
WHERE orderid= 167 LIMIT 1

我之前已經完成了子查詢更新，但是返回的mysql錯誤是：

您不能在FROM子句中指定目標表“ tbl_orders”進行更新

誰能看到我在做什么錯？

Answer 1

JOIN吧。

UPDATE  tbl_orders a
        INNER JOIN
        (
            SELECT  orderid, quantity * cost totalCost
            FROM    tbl_orders 
            WHERE   orderid = 167
        ) b ON a.orderid = b.orderid
SET     a.total = b.totalCost
WHERE   a.orderid = 167

如果要更新所有orderid ，請刪除所有WHERE子句。