mysql case then使用

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');

第一种用法:
SELECT name,
CASE WHEN birthday < '1981' THEN 'old' 
WHEN birthday > '1988' THEN 'yong'
ELSE 'ok' END YORN
FROM lee

mysql case then使用

第二种用法:
SELECT NAME, CASE name
WHEN 'sam' THEN 'yong'
WHEN 'lee' THEN 'handsome'
ELSE 'good' END as oldname
FROM lee

mysql case then使用

第三种: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;

mysql case then使用

在这里用sql语句进行日期比较的话,需要对年加引号,要不然可能结果和预期的结果不同, 
当然也可以用year函数来实现
select name,
case when year(birthday) > 1988 then 'yong'
when year(birthday) < 1980 then 'old'
else 'ok' END
from lee;

mysql case then使用

==========================================================
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);

第一题: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

select payment_date, amount,
case 
when amount >= 0 AND amount < 40 then 'low'
when amount >=40 AND amount < 80 then 'moderate'
when amount >=80 then 'high' 
else 'null' END
FROM penalties

第二题:统计出属于low的罚款编号
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'

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