SQL Server中使用CASE WHEN THEN语句

Question

我有一个简单的查询是这样的：

select t1.name,t1.bday,t2.address,t2.contactnum 
from table1 as t1 
left join table2 as t2 on t1.p_id = t2.p_id 
where (case when @qualified = '2' then t2.role is null 
     case when @qualified = '3' then t2.role is not null` end) 

当我执行查询时出现错误弹出表示：

不正确的语法关键字附近“是”。

任何想法为这家伙工作？

谢谢！

此查询的目的是获取表中的空行和非空行，具体取决于参数@qualified上传递的值。

Answer 1

试试这个：

select t1.name,t1.bday,t2.address,t2.contactnum 
from table1 as t1 
left join table2 as t2 on t1.p_id = t2.p_id 
where (@qualified = '2' AND t2.role is null) OR (@qualified = '3' AND t2.role is not null) 

我相信这句法表示您试图实现的条件表达式。但是，这样的WHERE子句可能会导致性能问题。如果这会发生，你应该使用：

IF @qualified = '2' THEN 
BEGIN 
    select t1.name,t1.bday,t2.address,t2.contactnum 
    from table1 as t1 
    left join table2 as t2 on t1.p_id = t2.p_id 
    where t2.role is null 
END 

IF @qualified = '3' THEN 
BEGIN 
    select t1.name,t1.bday,t2.address,t2.contactnum 
    from table1 as t1 
    left join table2 as t2 on t1.p_id = t2.p_id 
    where t2.role is not null 
END 

Answer 2

试试这个（未经测试）

SELECT t1.name,t1.bday,t2.address,t2.contactnum 
FROM table1 as t1 
LEFT JOIN table2 AS t2 ON t1.p_id = t2.p_id 
WHERE 
    CASE @qualified 
     WHEN '2' THEN t2.role is null 
     WHEN '3' THEN t2.role is not null 
    END 

Answer 3

语法不正确： 请看http://msdn.microsoft.com/en-IN/library/ms181765.aspx

还张贴您确切的要求，使得其容易建议如何使用CASE。

Answer 4

请尝试：

select t1.name,t1.bday,t2.address,t2.contactnum 
from table1 as t1 
left join table2 as t2 on t1.p_id = t2.p_id 
where (case when t2.role is null then '2' else '3' end)=@qualified