关于case when的用法

oracle case when 语句的用法详解

 

1. CASE WHEN 表达式有两种形式

复制代码代码如下:

--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

复制代码代码如下:

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 用法

复制代码代码如下:

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 用法

复制代码代码如下:

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; 

--摘自L蓝天碧水L博客