Question

问题：我有一个库存表和一个列出正在拍卖的物品的表格。我想要一个别名字段（“isAuction”）来表示拍卖品表中是否存在具有库存库存号的商品。

我写了以下查询：

SELECT FROM inventory AS i
EXISTS(SELECT * FROM auctionitems AS a WHERE a.stockNo = i.stockNo) AS "isAuction"

这会根据需要填充“isAuction”字段，其中包含1或0。

我现在添加一个WHERE条件：

SELECT FROM inventory AS i
EXISTS(SELECT * FROM auctionitems AS a WHERE a.stockNo = i.stockNo) AS "isAuction" 
WHERE isAuction = 1

但是，当我添加一个WHERE条件时，我收到一个错误： “＃1054 - 'where子句'中的未知列'isAuction'”

两个问题： 1）我在哪里弄错了？ 2）有没有更好的方法来解决我的问题？

Answer 1

SELECT i.*
FROM inventory AS i
WHERE EXISTS(SELECT * FROM auctionitems AS a WHERE a.stockNo = i.stockNo)

Answer 2

尝试HAVING isAuction = 1。

我在某处读到HAVING比WHERE更清醒的计算列。

Answer 3

您无法在WHERE中使用别名。你可以做到

WHERE EXISTS(SELECT * FROM auctionitems AS a WHERE a.stockNo = i.stockNo)

或者您可以将条件放入HAVING子句（mysql允许您使用HAVING而不使用聚合函数）： HAVING isAuction = 1 另外，你可以写

SELECT * FROM 
(SELECT i.*,
 EXISTS(SELECT * FROM auctionitems AS a WHERE a.stockNo = i.stockNo) AS isAuction 
 FROM inventory AS i

)a
WHERE a.isAuction = 1

MySQL：SELECT EXISTS（）AS字段WHERE field = x

3 个答案: