如何编写具有多种情况的 where 子句？

Question

我有一个可以为空的参数的存储过程，下面是我的查询

select * 
from table1
where table1.id = isnull(@id, table1.id)

现在有一些特殊的 id 我会区别对待。我正在添加另一个表 table2，如下所示

combid    id
1         abc01
1         abc02
1         abc03
2         hig01
2         hig02

我必须更改查询以满足以下情况

1. 如果@id 为空，where 子句将为table1.id = table1.id
2. 如果@id不为空，
   2.1 如果table2中存在@id，则where子句为table1.id in (select id from table2 where combid in (select combid from table2 where id=@id))
   2.2 否则where子句将是table1.id = @id

我尝试了以下查询，但不起作用。

select  * from table1
where (table1.id=@id and not exists(select * from table2 where id=@id)
or @id is null
or table1.id in (select id from table2 where combid in (select combid where id=@id)) and  exists(select * from table2 where id=@id))

如何更改存储过程的查询？

Answer 1

SELECT * FROM table1
WHERE
    @id IS NULL
    OR
    (
        @id IS NOT NULL
        AND
        (      
            (
                EXISTS (SELECT * FROM TABLE2 WHERE id = @id)
                AND
                table1.id IN (SELECT id FROM table2 WHERE combid in (SELECT combid FROM table2 WHERE id=@id))
            ) 
            OR
            (
                NOT EXISTS (SELECT * FROM TABLE2 WHERE id = @id)
                AND         
                table1.id = @id
            )
        )
    )

Answer 2

您可以使用 if...else... 而不是只编写一个 SELECT 语句，对您的案例使用 if else 更容易阅读。

if(@id is null)
    select * from table1
else if exists (select 1 from table2 where id = @id)
    select * from table1 
    where table1.id in 
                  (select id from table2 where combid in 
                         (select combid from table2 where id=@id)
                  )
else 
   select * from table1 where table1.id=@id