CASE WHEN表达式

1. CASE WHEN 表达式有两种形式

--简单Case函数  

CASE 易做图  
WHEN '1' THEN '男'  
WHEN '2' THEN '女'  
ELSE '其他' END  

--Case搜索函数  

CASE
WHEN 易做图 = '1' THEN '男'  
WHEN 易做图 = '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 易做图 = 1 THEN 1      /*易做图 1为男生，2位女生*/
                       ELSE NULL
                       END) 男生数,
                COUNT (CASE WHEN 易做图 = 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(易做图, '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易做图殊实现

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%')