case when then的用法

用法一：等值判断，相当于switch

CASE expression
　　WHEN value1 THEN returnValue1
　　WHEN value2 THEN returnValue2
　　WHEN value3 THEN returnValue3
　　ELSE defaultValue
END

 例子：论坛中用户的等级

create table [user]
(uId int identity(1,1) primary key,name varchar(50),level int  --1骨灰 2大虾 3菜鸟
)
insert into [user] (name,level) values('犀利哥',1)
insert into [user] (name,level) values('小月月',2)
insert into [user] (name,level) values('芙蓉姐姐',3)select name,等级=(case levelwhen 1 then '骨灰'when 2 then '大虾'when 3 then '菜鸟'end
) 
from [user]

用法二：区间判断，相当于if…else if…else…

CASE
　　WHEN condition1 THEN returnValue1
　　WHEN condition 2 THEN returnValue2
　　WHEN condition 3 THEN returnValue3
　　ELSE defaultValue
END

例子：

select studentId,成绩=( casewhen english between 90 and 100 then 'A'when english between 80 and 89 then 'B'when english between 70 and 79 then 'C'when english between 60 and 69 then 'D'when english < 60 then 'E'else '缺考'end
)
from score

注意：then后面返回的数据类型要一致， returnValue1、 returnValue2、 returnValue3的数据类型必须一致。