按2个日期字段排序

我会假设一些东西union就足够了,因为没有上下文我只能假设这是你正在寻找的东西？

SELECT t.id, t.date_in AS alias FROM test t
UNION ALL
SELECT t2.id, t2.date_out AS alias FROM test t2
ORDER BY alias ASC;

会给你一个如下结果:

+----+---------------------+
| id | alias               |
+----+---------------------+
|  2 | 2016-01-01 12:00:00 |
|  1 | 2016-01-01 12:00:00 |
|  2 | 2016-01-02 12:00:00 |
|  1 | 2016-01-03 12:00:00 |
+----+---------------------+
4 rows in set (0.00 sec)