转载自：http://www.cnblogs.com/eshizhan/archive/2012/04/06/2435493.html

CASE WHEN 表达式有两种形式

--简单Case函数  

CASE sex  
WHEN '1' THEN '男'  
WHEN '2' THEN '女'  
ELSE '其他' END  

--Case搜索函数  

CASE
WHEN sex = '1' THEN '男'  
WHEN sex = '2' THEN '女'  
ELSE '其他' END

CASE WHEN 在语句中不同位置的用法

1.SELECT CASE WHEN 用法

SELECT grade,COUNT (CASE WHEN sex = 1 THEN 1 /*sex 1为男生，2位女生*/ ELSE NULL END) 男生数,COUNT (CASE WHEN sex = 2 THEN 1 ELSE NULL END) 女生数 FROM students GROUP BY grade;

2.WHERE CASE WHEN 用法

SELECT T2.*,T1.* FROM T1,T2 WHERE (CASE WHEN T2.COMPARE_TYPE = 'A' AND T1.SOME_TYPE LIKE 'NOTHING%' THEN 1 WHEN T2.COMPARE_TYPE != 'A' AND T1.SOME_TYPE NOT LIKE 'NOTHING%' THEN 1 ELSE 0 END) = 1

3.GROUP BY CASE WHEN 用法

SELECT CASE WHEN salary <= 500 THEN '1' WHEN salary > 500 AND salary <= 600 THEN '2' WHEN salary > 600 AND salary <= 800 THEN '3' WHEN salary > 800 AND salary <= 1000 THEN '4' ELSE NULL END salary_class,-- 别名命名 COUNT(*) FROM Table_A GROUP BY CASE WHEN salary <= 500 THEN '1' WHEN salary > 500 AND salary <= 600 THEN '2' WHEN salary > 600 AND salary <= 800 THEN '3' WHEN salary > 800 AND salary <= 1000 THEN '4' ELSE NULL END;

关于IF-THEN-ELSE的其他实现

1.DECODE() 函数

select decode(sex,'M','Male','F','Female','Unknown') from employees;

貌似只有Oracle提供该函数，而且不支持ANSI sql，语法上也没CASE WHEN清晰，个人不推荐使用。

2.在WHERE中特殊实现

SELECT T2.*,T2 WHERE (T2.COMPARE_TYPE = 'A' AND T1.SOME_TYPE LIKE 'NOTHING%') OR (T2.COMPARE_TYPE != 'A' AND T1.SOME_TYPE NOT LIKE 'NOTHING%')

这种方法也是在特殊情况下使用，要多注意逻辑，不要弄错。