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详解Mysql case then使用

程序员文章站 2024-02-24 21:33:04
表的创建 create table `lee` ( `id` int(10) not null auto_increment, `name` char(20...

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

第二种用法:

select name, case name
 when 'sam' then 'yong'
 when 'lee' then 'handsome'
 else 'good' end as oldname
from lee

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;

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;
==========================================================
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'

ps:mysql,case when,case多个字段

select distinct a.patientid,a.patientcode,a.patientsex,a.mobileno,a.homephoneno,a.userage,a.patientname,a.patientidcard, date_format(a.registdate,'%y-%m-%d') as registdate, 
 case when b.usedstarttime is not null and b.usedendtime is null then '1'
when b.usedstarttime is not null and b.usedendtime is not null then '2' 
 end as 'usedstate'
 from mets_v_patient_baseinfo a 
 left join mets_devices_used_history b on a.patientid = b.patientid
 where  (select ifnull(isdeleted,0) from userpublic_info where userid = a.patientid ) = 0 
 and 1=1 
 order by patientid desc limit 0,15