Oracle學習筆記_06_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搜尋函數  

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      /*sex 1為男生，2位女生*/
                    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, -- 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.關于IF-THEN-ELSE的其他實作

3.1 DECODE() 函數

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

貌似隻有Oracle提供該函數，而且不支援ANSI SQL，文法上也沒CASE WHEN清晰，個人不推薦使用。

3.2 在WHERE中特殊實作

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。

附錄：參考資料

1.

Oracle CASE WHEN 用法介紹

2.

https://dba.stackexchange.com/questions/1170/oracle-sql-case-in-a-where-clause