CASE语句内部存在

Question

我需要创建一个查询，看起来像这样：

SELECT attr
FROM SomeTable B
WHERE EXISTS
    (CASE WHEN B.some_attr = 0
        THEN (SELECT x FROM C WHERE B.z = z)
     WHEN B.some_attr = 1
        THEN (SELECT x FROM D WHERE B.z = x)
     END)

但是SQL Server将拒绝执行此带来一些语法错误。
如何修改此查询以使其工作？

编辑：我加入了错误：

Msg 156, Level 15, State 1, Line 4 Incorrect syntax near the keyword 'CASE'.
Msg 156, Level 15, State 1, Line 6 Incorrect syntax near the keyword 'WHEN'.
Msg 102, Level 15, State 1, Line 8 Incorrect syntax near ')'.

Answer 1

试试这个：

SELECT A 
FROM B 
WHERE 
    CASE 
    WHEN B.some_attr = 0 AND EXISTS(SELECT x FROM C WHERE B.z = z) THEN 1 
    WHEN B.some_attr = 1 AND EXISTS(SELECT x FROM D WHERE B.z = x) THEN 1 
    END = 1 

Answer 2

试试这个：

SELECT A 
FROM B 
WHERE 
    (CASE WHEN B.some_attr = 0 
      THEN (SELECT x FROM C WHERE B.z = z) 
      WHEN B.some_attr = 1 
      THEN (SELECT x FROM D WHERE B.z = x) 
    END) is not null 

这是假设子查询返回一行。如果不是，只需放入max（x）而不是x。

Answer 3

我认为这应该工作以及：

SELECT attr 
FROM SomeTable B 
WHERE EXISTS (
    SELECT 1 FROM C WHERE B.some_attr = 0 AND B.z = z UNION ALL 
    SELECT 1 FROM D WHERE B.some_attr = 1 AND B.z = x 
)