oracle case when 語句的用法詳解

1. CASE WHEN 運算式有兩種形式
複製代碼 代碼如下:--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
複製代碼 代碼如下: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;

2.2 WHERE CASE WHEN 用法
複製代碼 代碼如下: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

2.3 GROUP BY CASE WHEN 用法
複製代碼 代碼如下: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
複製代碼 代碼如下: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
複製代碼 代碼如下: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。