CASE函数

-> 使用类似switch-case与if-else if

-> 语法

•case [字段]

•    when 表达式 then 显示数据

•    when 表达式 then 显示数据

•    else 显示数据

•end as 别名

-> then 后数据类型要一致

 

简单的例子:

 

select 
    case 
        when Sex='m' then '男'
        when Sex='f' then '女' 
        else '中性'
    end as 性别
    ,*
from dbo.StudentTestInsert

select 
    case Sex
        when 'm' then '男'
        when 'f' then '女'
        else '中性'
    end as 性别
    ,*
from Student

select 
    case 
        when Score >= 90 then 'A'
        when Score >=80  then 'B'
        when Score >=70  then 'C'
        when Score >=60  then 'D'
        else 'E'
    end as Score1
    ,case Score/10
        when 10 then 'A'
        when 9  then 'A'
        when 8  then 'B'
        when 7  then 'C'
        when 6  then 'D'
        else 'E'
    end as Score2
from Score 

create table Score
(
    学号 nvarchar(10),
    课程 nvarchar(10),
    成绩 int
)

insert into Score values('0001','语文',87);
insert into Score values('0001','数学',79);
insert into Score values('0001','英语',95);
insert into Score values('0002','语文',69);
insert into Score values('0002','数学',84);


select 
    学号, 
    SUM(case when 课程='语文' then 成绩 else 0 end) as 语文, 
    SUM(case when 课程='数学' then 成绩 else 0 end) as 数学, 
    AVG(case when 课程='英语' then 成绩 end) as 英语 
from 
    Score 
group by 
    学号;

2 select 
    学号, 
    case when 课程='语文' then 成绩 else 0 end as 语文, 
    case when 课程='数学' then 成绩 else 0 end as 数学, 
    case when 课程='英语' then 成绩 end as 英语 
from 
    Score