Question

我有一个SQL查询，看起来像这样：

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，field3显示0，尽管2 - 1 - 0 = 1。

使用COALESCE，因为我左外连接几个表，这可能会导致Null值。

如果field3为null，我也会遇到任何问题吗？

谢谢

Answer 1

独立合并：

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field2实际上是NULL而不是0。

Answer 2

您无法使用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将主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操作顺序如下：

FROM clause
WHERE子句
GROUP BY子句
HAVING条款
SELECT条款
ORDER BY子句

ALIAS在SELECT语句中提供，并且在创建它们的级别上无法访问。

从COALESCE中减去

2 个答案: