case when 使用小结

1、等值转换

2、范围转换

3、行转列

 

1、等值转换  性别转换

select
name as '名字',
(case sex when 0 then '女' else '男' end) as '性别'
from test.student;

2、范围转换

有的时候，也会遇到这种情况，按照用户成绩显示优（90+）、良（80-90）、及格（60-80）、未及格（60-）

select 
name as '姓名'
,(case score when score>=90 then '优' when score>=80 then '良' when score>=60 then '及格' else '不及格' end) as '等级'
from test.stu_score; 

 

3、行转列

第一步 先按照科目分开， 符合条件的设置分数，不符合的给置零。

select name as '姓名'
,(case course when '语文' then score else 0 end) as '语文'
,(case course when '数学' then score else 0 end) as '数学'
,(case course when '英语' then score else 0 end) as '英语'
from test.course_score
 

 

然后再按照名字group by ，对分数求max。

select name as '姓名'
,max(case course when '语文' then score else 0 end) as '语文'
,max(case course when '数学' then score else 0 end) as '数学'
,max(case course when '英语' then score else 0 end) as '英语'
from test.course_score group by name;

本文地址：https://blog.csdn.net/m0_38017766/article/details/107639561