mysql - 从 COALESCE 中减去

Question

我有一个看起来像这样的 SQL 查询：

SELECT 
COALESCE(field1, 0) AS field1, 
COALESCE(field2, 0) AS field2,
COALESCE(field3 - field1 - field2, 0) AS field
FROM ...

However the problem with it is that given say:

field1 = 1
field2 = 0
field3 = 2

显示输出时，字段 3 显示 0，尽管 2 - 1 - 0 = 1。

COALESCE 用于我离开外部加入一些可能导致 Null 值的表。

如果 field3 为空，我也会有任何问题吗？

谢谢

Answer 1

独立进行合并：

SELECT COALESCE(field1, 0) AS field1, COALESCE(field2, 0) AS field2,
       COALESCE(field3, 0) - coalesce(field1, 0) - coalesce(field2, 0) AS field
FROM ...

至于你的例子，我怀疑这field2实际上是NULL而不是 0。

Answer 2

您不能使用在同一级别上给出的别名进行计算。您需要再次重复该函数或仅将主要计算包装在子查询中，

SELECT  field1, field2,
        field3 - (field1 + field2)
FROM
        (
            SELECT  COALESCE(field1, 0) AS field1, 
                    COALESCE(field2, 0) AS field2, 
                    COALESCE(field3, 0) field3
            FROM    TableName
        ) x

原因是因为 SQL 操作的顺序如下：

• FROM 子句
• WHERE 子句
• GROUP BY 子句
• HAVING 子句
• 选择子句
• ORDER BY 子句

ALIAS在SELECT声明中给出，并且在它们创建的级别上无法访问。