oracle case用法

Posted 2020-10-12 点点滴滴

Oracle CASE WHEN 用法介绍

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%\')