oracle中case when的使用

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

1. case when表达式的两种形式：

--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sql语句不同位置的用法：

2.1 select case when

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;

2.2 where case when

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

2.3 group by case when

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. if-then-else的其他实现

3.1 decode函数：

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