解释：

SELECT            
    case                   ————-如果
    when sex=’1′ then ‘男’ ————-sex=’1’，则返回值’男’
    when sex=’2′ then ‘女’ ————-sex=’2’，则返回值’女’  
    else 0                 ————-其他的返回’其他’
    end                    ————-结束
from   sys_user            ——–整体理解： 在sys_user表中如果sex=’1’，则返回值’男’如果sex=’2’，则返回值’女’ 否则返回’其他’

—用法一：
SELECT
            CASE WHEN STATE = ‘1’ THEN ‘成功’
                 WHEN STATE = ‘2’ THEN ‘失败’
            ELSE ‘其他’ END  
            FROM  SYS_SCHEDULER
—用法二：    
SELECT STATE
            CASE WHEN ‘1’ THEN ‘成功’
                 WHEN ‘2’ THEN ‘失败’
            ELSE ‘其他’ END  
            FROM  SYS_SCHEDULER

列子：

有员工表empinfo
(
Fempno varchar2(10) not null pk,
Fempname varchar2(20) not null,
Fage number not null,
Fsalary number not null
);
假如数据量很大约1000万条；写一个你认为最高效的SQL，用一个SQL计算以下四种人：
fsalary>9999 and fage > 35
fsalary>9999 and fage < 35
fsalary <9999 and fage > 35
fsalary <9999 and fage < 35
每种员工的数量；
select sum(case when fsalary > 9999 and fage > 35
then 1
else 0end) as “fsalary>9999_fage>35”,
sum(case when fsalary > 9999 and fage < 35
then 1
else 0
end) as “fsalary>9999_fage<35”,
sum(case when fsalary < 9999 and fage > 35
then 1
else 0
end) as “fsalary<9999_fage>35”,
sum(case when fsalary < 9999 and fage < 35
then 1
else 0
end) as “fsalary<9999_fage<35”
from empinfo;