SQL Server 使用 CASE WHEN THEN 语句

Question

我有一个像这样的示例查询:

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，会弹出错误，指示:

Incorrect syntax near the keyword 'is'.

有什么办法可以解决这些人的问题吗？

谢谢!

此查询的目的是根据参数 @qualified 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您试图实现的条件表达式。但是，此类 WHERE 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