現在有一個表，欄位age和type，type = 1的時候需要age降序排列，type = 2的時候age升序排列。一開始覺得很簡單啊，於是就寫：

SELECT * FROM table_a ORDER BY CASE WHEN type = 1 THEN 'age DESC' WHEN type = 2 THEN 'age ASC' END

後來發現，根本不行！於是又換方法

SELECT * FROM table_a ORDER BY IF(type = 1,'age DESC','age ASC')

然鵝又失敗了。於是在朋友提點下，最終解決辦法：

方法一：使用union all

SELECT * FROM table_a WHERE type = 1 ORDER BY age DESC
UNION ALL
SELECT * FROM table_a WHERE type = 2 ORDER BY age ASC

方法二：當type = 2的時候把age欄位變成負數，然後全部降序排序

SELECT type
,(CASE WHEN type = 1 THEN age * 1 WHEN type = 2 THEN age * -1 END) field
FROM table_a
ORDER BY field DESC