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 '其餘' END --Case搜索函數   CASE WHEN sex = '1' THEN '男' WHEN sex = '2' THEN '女' ELSE '其餘' END

 

2. CASE WHEN 在語句中不一樣位置的用法

2.1 SELECT CASE WHEN 用法

SELECT grade, COUNT (CASE WHEN sex = 1 THEN 1 /*sex 1爲男生，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的其餘實現

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，並且不支持ANSI SQL，語法上也沒CASE WHEN清晰，我的不推薦使用。

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%')