Question

我试图根据结果数量在两个选择的staments之间进行选择。在以下case语句之外进行测试时，每个语句都返回有效数据。

SELECT CASE 
         WHEN Count(base.id) > 0 THEN base.id 
         ELSE (SELECT id 
               FROM   tablea 
               WHERE  pid = @val) 
       END 
FROM   (SELECT id 
        FROM   tablea 
        WHERE  id = @val) AS base

当base.Id> 0时，它返回正确，否则为Null，而它应该从我的表中有7行

Answer 1

WHEN COUNT(base.id) >0始终为真，因为您的FROM会过滤掉id＆lt;＆gt; @val的行。除非我误解了你想要的东西，否则以下内容会给你带来理想的结果：

SELECT id               
FROM   tablea 
WHERE  id = @val OR pid = @val

更新：

SELECT id               
FROM   tablea 
WHERE  id = @val OR (pid = @val 
AND NOT EXISTS (SELECT NULL FROM tablea WHERE id = @val))

Answer 2

听起来你想要选择一组行，否则就会选择第二组。一种方法是union两组，然后选择你想要的东西：

select id from (
  select id, 1 as Selector
    from tablea
    where id = @val
  union
  select id, 2
    from tablea
    where pid = @val ) as Placeholder
  where Selector = case when exists ( select 42 from tablea where id = @val ) then 1 else 2 end

我假设id不可为空。

Sql案例没有返回值

2 个答案: