Oracle CASE WHEN 用法介紹

1. CASE WHEN 表達式有兩種形式

--簡單Case函數  


CASE sex  

WHEN '1' THEN '男'  

WHEN '2' THEN '女'  

ELSE '其他' END  



--Case搜索函數  


CASE

WHEN sex = '1' THEN '男'  

WHEN sex = '2' THEN '女'  

ELSE '其他' END  

2. CASE WHEN 在語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      

                       ELSE NULL

                       END) 男生數,

                COUNT (CASE WHEN sex = 2 THEN 1

                       ELSE NULL

                       END) 女生數

    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, -- 別名命名
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.關於IF-THEN-ELSE的其他實現

3.1 DECODE() 函數

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

from   employees;

 

貌似只有Oracle提供該函數，而且不支持ANSI SQL，語法上也沒CASE WHEN清晰，個人不推薦使用。

3.2 在WHERE中特殊實現

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%')

這種方法也是在特殊情況下使用，要多注意邏輯，不要弄錯。