mysql - 如何使用 GROUP BY 以便按特定列排序？

Question

我需要从下表中获取所有最新入住时间和退房时间的列表:

Table: checkin_times

    +----------+-------------------------+---------------------+
    |  Person  | in_time                 | out_time            |
    +----------+-------------------------+---------------------+
    |  Bob     | 2015-08-10 07:15:00     | 2015-08-10 17:15:00 |
    |  Bob     | 2015-08-12 07:00:00     | NULL                |
    |  Geoff   | 2015-08-12 07:40:00     | 2015-08-12 17:40:00 |
    |  Bob     | 2015-08-11 07:15:00     | 2015-08-12 17:15:00 |
    |  Geoff   | 2015-08-10 07:00:00     | 2015-08-12 17:00:00 |
    +----------+-------------------------+---------------------+

(显然 NULL 表示此人尚未 checkout 。)

我对最新条目不感兴趣。因此，对于这个例子，输出应该是:

+----------+-------------------------+---------------------+
|  Person  | in_time                 | out_time            |
+----------+-------------------------+---------------------+
|  Bob     | 2015-08-12 07:00:00     | NULL                |
|  Geoff   | 2015-08-12 07:40:00     | 2015-08-12 17:40:00 |
+----------+-------------------------+---------------------+

我尝试了以下 SQL:

SELECT person, MAX(in_time) AS in_time, MAX(out_time) AS out_time 
FROM checkin_times  
GROUP BY person
ORDER BY person;

但它无法正确处理 NULL，而是返回 out_time 的最新实际日期。

理想情况下，我希望 GROUP BY 的顺序仅在确定顺序时才考虑 in_time。

Answer 1

如果我理解正确，您正在尝试获取每个人最新 in_time 的所有列值。

然后首先获取最新的in_time并将它们连接到所有记录上。

示例:

mysql> select ct.person, ct.in_time, ct.out_time
    ->   from checkin_times ct
    ->   right join ( select person, max( in_time ) as in_time
    ->                 from checkin_times group by person ) it
    ->          on ct.person = it.person and ct.in_time=it.in_time;
+--------+---------------------+---------------------+
| person | in_time             | out_time            |
+--------+---------------------+---------------------+
| Bob    | 2015-08-12 07:00:00 | NULL                |
| Geoff  | 2015-08-12 07:40:00 | 2015-08-12 17:40:00 |
+--------+---------------------+---------------------+
2 rows in set (0.04 sec)

SQL Fiddle Demo