CASE When的用法

一、case when主要用来转换显示

1.1、用法一case when搜索函数

数据库查询结果是woman和man，转为男和女

SELECT
CASE WHEN SEX = 'WOMAN' THEN '女' 
	 WHEN SEX = 'MAN' THEN '男' END 
FROM
	 T_SYS_USER
	 
注：给转换后的列起别名：
	CASE WHEN SEX = 'WOMAN' THEN '女' 
	 WHEN SEX = 'MAN' THEN '男' END as 性别

结果展示：

1.2 case的简单函数

简单函数形式：case 列名 when ‘条件’ then ‘显示结果’

SELECT
CASE SEX WHEN 'WOMAN' THEN '女' 
		 WHEN 'MAN' THEN '男' 
		 ELSE '其他' END as 性别
FROM
		T_SYS_USER 

1.3 case when放在where后面需要注意的情况

在where后面这样使用将会报错：

select * 
from t1, t2
where t2.name in
CASE
    when t1.id = 0 then ('aaa', 'bbb')
    when t1.id = 1 then ('bbb', 'ccc')
    when t1.id = 2 then ('ccc', 'ddd')
END

正确写法：

select * 
from t1, t2
where t2.name in
CASE
    when t1.id = 0 then B.name in ('aaa', 'bbb')
    when t1.id = 1 then B.name in ('bbb', 'ccc')
    when t1.id = 2 then B.name in ('ccc', 'ddd')
END

加深对case when和where的理解：case when放置到where后面