解釋：

1 SELECT            
2     case                   -------------如果
3     when sex=‘1‘ then ‘男‘ -------------sex=‘1‘，則返回值‘男‘
4     when sex=‘2‘ then ‘女‘ -------------sex=‘2‘，則返回值‘女‘  
5     else ‘其他‘                 -------------其他的返回‘其他’
6     end                    -------------結束
7 from   sys_user            --
 
------整體理解： 在sys_user表中如果sex=‘1‘，則返回值‘男‘如果sex=‘2‘，則返回值‘女‘ 否則返回‘其他’

--- 用法一：

1 SELECT 
2             CASE WHEN STATE = ‘1‘ THEN ‘成功‘ 
3                  WHEN STATE = ‘2‘ THEN ‘失敗‘
4             ELSE ‘其他‘ END  
5             FROM  SYS_SCHEDULER

---用法二：

1 SELECT STATE
2             CASE WHEN ‘1
 
‘ THEN ‘成功‘ 
3                  WHEN ‘2‘ THEN ‘失敗‘
4             ELSE ‘其他‘ END  
5             FROM  SYS_SCHEDULER

例子：

 1 有員工表empinfo 
 2 ( 
 3 Fempno varchar2(10) not null pk, 
 4 Fempname varchar2(20) not null, 
 5 Fage number not null, 
 6 Fsalary number not null 
 7 ); 
 8 假如數據量很大約1000萬條；寫一個你認為最高效的SQL，用一個SQL計算以下四種人： 
 
 9 fsalary>9999 and fage > 35 
10 fsalary>9999 and fage < 35 
11 fsalary <9999 and fage > 35 
12 fsalary <9999 and fage < 35 
13 每種員工的數量； 
14 select sum(case when fsalary > 9999 and fage > 35
15 then 1
16 else 0end) as "fsalary>9999_fage>35",
17 sum(case when fsalary > 9999 and fage < 35
18 then 1
19 else 0
20 end) as "fsalary>9999_fage<35",
21 sum(case when fsalary < 9999 and fage > 35
22 then 1
23 else 0
24 end) as "fsalary<9999_fage>35",
25 sum(case when fsalary < 9999 and fage < 35
26 then 1
27 else 0
28 end) as "fsalary<9999_fage<35"
29 from empinfo;

MySql 中關鍵字 case when then else end 的用法