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详解Mysql case then使用

作者:简单--生活

mysql case then在程序开发中经常用到,通过本文给大家介绍mysql case then使用相关知识,对mysql case then相关知识感兴趣的朋友一起学习吧

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 

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