我想设置`amount`为`-1`如果不存在，用`-1`

Question

下一步比较我有这些记录：

Order 
---------- 
id amount 
1 10 
2 NULL 
3 20 

我想设置amount到-1如果不存在，下一步与-1比较。

我写这篇文章：

SELECT 
    id, 
    COALESCE(o.amount, -1) AS amount2 
FROM Order o 
WHERE amount2 = -1 

，但这个错误：

#1054 - Unknown column 'amount2' in 'where clause' 

我要退货：

Order 
---------- 
id amount2 
2 -1 

Answer 1

你不能在WHERE使用列别名条款。

试试这个：

SELECT 
    id, 
    COALESCE(o.amount, -1) AS amount2 
FROM Order o 
WHERE COALESCE(o.amount, -1) = -1 

An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column

从这里引：http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html

Answer 2

我认为，

SELECT ID，AMOUNT2 FROM（SELECT ID，COALESCE（金额，-1）AMOUNT2 FROM order）n WHERE n.amount2 = -1

好看！