mysql的case when用法

表的创建

CREATE TABLE `lee` (
`id` int(10) NOT NULL AUTO_INCREMENT,
`name` char(20) DEFAULT NULL,
`birthday` datetime DEFAULT NULL,
PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8

数据插入：

insert into lee(name,birthday) values ('sam','1990-01-01');

insert into lee(name,birthday) values ('lee','1980-01-01');

insert into lee(name,birthday) values ('john','1985-01-01');

使用case when语句

1、

select name,
case
when birthday<'1981' then 'old'
when birthday>'1988' then 'yong'
else 'ok' END yorn
from lee;

2、

select NAME,
case name
when 'sam' then 'yong'
when 'lee' then 'handsome'
else 'good' end
from lee;

3、当然了case when语句还可以复合

select name,birthday,
case
when birthday>'1983' then 'yong'
when name='lee' then 'handsome'
else 'just so so ' end
from lee;

由上可以看出，case when 语句实际上是根据原表查询出的作为一个字段存在的数据，可以在end 后指定字段名称，也可不指定

在这里用sql语句进行日期比较的话，需要对年加引号。要不然可能结果可能和预期的结果会不同。我的mysql版本5.1

当然也可以用year函数来实现，以第一个sql为例

select NAME,
CASE
when year(birthday)>1988 then 'yong'
when year(birthday)<1980 then 'old'
else 'ok' END
from lee;



create table penalties
(
paymentno INTEGER not NULL,
payment_date DATE not null,
amount DECIMAL(7,2) not null,
primary key(paymentno)
)

insert into penalties values(1,'2008-01-01',3.45);
insert into penalties values(2,'2009-01-01',50.45);
insert into penalties values(3,'2008-07-01',80.45);

1.#对罚款登记分为三类，第一类low，包括大于0小于等于40的罚款，第二类moderate大于40

#到80之间的罚款，第三类high包含所有大于80的罚款。

2.#统计出属于low的罚款编号。

第一道题的解法与上面的相同

select paymentno,amount,
case
when amount>0 and amount<=40 then 'low'
when amount>40 and amount<=80 then 'moderate'
when amount>80 then 'high'
else 'incorrect' end lvl
from `penalties`;

2.#统计出属于low的罚款编号。重点看这里的解决方法

方法1

select paymentno,amount
from `penalties`
where case
when amount>0 and  amount<=40 then 'low'
when amount>40 and amount<=80 then 'moderate'
when amount>80 then 'high'
else 'incorrect' end ='low';

方法2

select *
from (select paymentno,amount,
case
when amount>0 and amount<=40 then 'low'
when amount>40 and amount<=80 then 'moderate'
when amount>80 then 'high'
else 'incorrect' end lvl
from `penalties`) as p
where p.lvl='low';