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  CASEWHEN 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。