MySQL的CASE WHEN语句的几个使用实例

使用case when进行字符串替换处理

mysql> select * from sales;

9 rows in set (0.01 sec)

select name as name,
    case category
    when "holiday" then "seasonal"
    when "profession" then "bi_annual"
    when "literary" then "random" end as "pattern"
from sales;                   

9 rows in set (0.00 sec)

create table sales(
    num mediumint not null auto_increment, name char(20), winter int, spring int,
    summer int, fall int, category char(13), primary key(num)
) engine=myisam;

insert into sales value(1, 'java', 1067 , 200, 150, 267,'holiday');
insert into sales value(2, 'c',970,770,531,486,'profession');
insert into sales value(3, 'javascript',53,13,21,856,'literary');
insert into sales value(4, 'sql',782,357,168,250,'profession');
insert into sales value(5, 'oracle',589,795,367,284,'holiday');
insert into sales value(6, 'mysql',953,582,336,489,'literary');
insert into sales value(7, 'cplus',752,657,259,478,'literary');
insert into sales value(8, 'python',67,23,83,543,'holiday');
insert into sales value(9, 'php',673,48,625,52,'profession');

简单语句

select case when 10*2=30 then '30 correct'
   when 10*2=40 then '40 correct'
   else 'should be 10*2=20'
end as `result`;

多重表达式

select case 10*2
   when 20 then '20 correct'
   when 30 then '30 correct'
   when 40 then '40 correct'
   else 'no results'
end as `result`;

在select查询中使用case when

create table dvds (
   id smallint not null auto_increment primary key,
   name varchar(60) not null,
   numdisks tinyint not null default 1,
   ratingid varchar(4) not null,
   statid char(3) not null
) engine=innodb;
insert into dvds (name, numdisks, ratingid, statid)
values ('christmas', 1, 'nr', 's1'), ('doc', 1, 'g',  's2'), ('africa', 1, 'pg', 's1'), ('falcon', 1, 'nr', 's2'),
       ('amadeus', 1, 'pg', 's2'), ('show', 2, 'nr', 's2'), ('view', 1, 'nr', 's1'), ('mash', 2, 'r',  's2');
select name, ratingid as rating,
   case ratingid
      when 'r' then 'under 17 requires an adult.'
      when 'x' then 'no one 17 and under.'
      when 'nr' then 'use discretion when renting.'
      else 'ok to rent to minors.'
   end as policy
from dvds
order by name;

8 rows in set (0.01 sec)