求助，怎么在where条件中用case 语句？
demo:

declare @city varchar(20)

select id,city from tb where id=@id

后面加case语句 if(@city='全部') 后面不加何条件，
结果是【select id,city from tb where id=@id 】
else city = @city;
结果是【select id,city from tb where id=@id and city = @city】



注明： 实际情况select 语句很长很长。想通过一条语句解决该问题。不分开写。
[解决办法]

DECLARE @city VARCHAR(20)
DECLARE @id INT 
SELECT  id ,
        city
FROM    tb
WHERE   id = @id
        AND city = CASE WHEN @city = '全部' THEN city
                        ELSE @city
                   END