sql - SQL Server 使用 CASE WHEN THEN 语句

Question

我有一个这样的示例查询：

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，会弹出一个错误指示：

关键字“is”附近的语法不正确。

有什么想法可以解决这个问题吗？

谢谢！

此查询的目的是根据参数@qualified 上传递的值获取表中的空行和非空行。

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)

我相信这个语法代表了你试图实现的条件表达式。但是，这样WHERE的子句可能会导致性能问题。如果发生这种情况，您应该使用：

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：请查看http://msdn.microsoft.com/en-IN/library/ms181765.aspx

还要发布您的确切要求，以便轻松建议您如何使用 CASE。

Answer 4

请试试：

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