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1. 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  

2. CASE WHEN 在语句中不同位置的用法

2.1 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;

2.3 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

2.4 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;  

3.关于IF-THEN-ELSE的其他实现

3.1 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。

3.2 在WHERE中特殊实现

SELECT T2.*, T1.*    FROM T1, 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。

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