Oracle CASE WHEN 用法介紹

文章目錄

• 2. CASE WHEN 在語句中不同位置的用法

1. CASE WHEN 運算式有兩種形式

--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, -- 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。