在 mysql 中使用 group by 仅选择最后一个值

2021-12-27 00:00:00 group-by mysql

我有一张表格，其中包含有关参加某些活动的数据.我在表中有用户每次发送新考勤的考勤数据，信息如下:

I have one table with data about attendance into some events. I have in the table the data of the attendance everytime the user sends new attendance, the information is like this:

mysql> SELECT id_branch_channel, id_member, attendance, timestamp, id_member FROM view_event_attendance WHERE id_event = 782;
+-------------------+-----------+------------+------------+-----------+
| id_branch_channel | id_member | attendance | timestamp  | id_member |
+-------------------+-----------+------------+------------+-----------+
|              1326 |    131327 |        459 | 1363208604 |    131327 |
|              1326 |    131327 |        123 | 1363208504 |    131327 |
|              1326 |    131327 |          1 | 1363208459 |    131327 |
|              1326 |     93086 |          0 |       NULL |     93086 |
|              1326 |     93087 |          0 |       NULL |     93087 |
|              1326 |     93088 |          0 |       NULL |     93088 |
|              1326 |     93093 |          0 |       NULL |     93093 |
|              1326 |     99113 |          0 |       NULL |     99113 |
|              1326 |     99135 |          0 |       NULL |     99135 |
|              1326 |     99199 |          0 |       NULL |     99199 |
|              1326 |     99200 |          0 |       NULL |     99200 |
|              1326 |    131324 |          0 |       NULL |    131324 |
|              1326 |     85850 |          0 |       NULL |     85850 |
|              1326 |     93085 |          0 |       NULL |     93085 |
+-------------------+-----------+------------+------------+-----------+
14 rows in set (0.00 sec)

(这实际上是一个视图，因此一些字段为空).

(This is actually a view, for that reason some of the fields are null).

我可以通过 id_member 分组，因此每个成员只能获得一行(即，只有用户发送的最后一次出席).但是，当我这样做时，我收到了用户发送的第一个出席，而不是最后一个.

I can groupby id_member so I get only one row for every member (that is, only the last attendance the user sent). However, when I do it, I received the first attendance the user sent, not the last one.

mysql> SELECT id_branch_channel, id_member, attendance, timestamp, id_member FROM view_event_attendance WHERE id_event = 782 GROUP BY id_event,id_member;
+-------------------+-----------+------------+------------+-----------+
| id_branch_channel | id_member | attendance | timestamp  | id_member |
+-------------------+-----------+------------+------------+-----------+
|              1326 |    131327 |          1 | 1363208459 |    131327 |
|              1326 |     93086 |          0 |       NULL |     93086 |
|              1326 |    131324 |          0 |       NULL |    131324 |
|              1326 |     93087 |          0 |       NULL |     93087 |
|              1326 |     93088 |          0 |       NULL |     93088 |
|              1326 |     93093 |          0 |       NULL |     93093 |
|              1326 |     99113 |          0 |       NULL |     99113 |
|              1326 |     99135 |          0 |       NULL |     99135 |
|              1326 |     85850 |          0 |       NULL |     85850 |
|              1326 |     99199 |          0 |       NULL |     99199 |
|              1326 |     93085 |          0 |       NULL |     93085 |
|              1326 |     99200 |          0 |       NULL |     99200 |
+-------------------+-----------+------------+------------+-----------+
12 rows in set (0.00 sec)

我已经尝试添加 ORDER BY 子句，但它们根本不起作用......有什么想法吗?

I already tried to add ORDER BY clausules, but they are not working at all... any ideas?

提前致谢！

编辑:这是创建表格的脚本

CREATE OR REPLACE VIEW view_event_attendance 
    AS
        SELECT 
            tbl_event.id_event,
            tbl_member_event.id_member,
            tbl_event.id_branch_channel,
            tbl_member_event_attendance.id_member_event_attendance,
            IF(ISNULL(tbl_member_event_attendance.attendance), 0, tbl_member_event_attendance.attendance) AS attendance,
            tbl_member_event_attendance.timestamp
        FROM 
            tbl_event
            INNER JOIN 
                tbl_member_event ON tbl_member_event.id_event = tbl_event.id_event
                LEFT OUTER JOIN
                    tbl_member_event_attendance ON tbl_member_event_attendance.id_member_event = tbl_member_event.id_member_event
        ORDER BY 
            tbl_member_event_attendance.timestamp DESC;

编辑 2:

非常感谢 MichaelBenjamin，但是使用子查询时的问题是视图的大小:

Thanks a lot MichaelBenjamin, but the problem when using subqueries is the size of the view:

mysql> DESCRIBE SELECT id_branch_channel, id_member, attendance, timestamp, id_member 
    -> FROM (select * from view_event_attendance order by timestamp desc) as whatever
    -> WHERE id_event = 782 
    -> GROUP BY id_event,id_member;
+----+-------------+-----------------------------+--------+-----------------+-----------------+---------+------------------------------------------------+-------+----------------------------------------------+
| id | select_type | table                       | type   | possible_keys   | key             | key_len | ref                                            | rows  | Extra                                        |
+----+-------------+-----------------------------+--------+-----------------+-----------------+---------+------------------------------------------------+-------+----------------------------------------------+
|  1 | PRIMARY     | <derived2>                  | ALL    | NULL            | NULL            | NULL    | NULL                                           | 16755 | Using where; Using temporary; Using filesort |
|  2 | DERIVED     | tbl_member_event            | index  | id_event        | id_event        | 8       | NULL                                           | 16346 | Using index; Using temporary; Using filesort |
|  2 | DERIVED     | tbl_event                   | eq_ref | PRIMARY         | PRIMARY         | 4       | video_staging.tbl_member_event.id_event        |     1 |                                              |
|  2 | DERIVED     | tbl_member_event_attendance | ref    | id_event_member | id_event_member | 4       | video_staging.tbl_member_event.id_member_event |     1 | Using index                                  |
+----+-------------+-----------------------------+--------+-----------------+-----------------+---------+------------------------------------------------+-------+----------------------------------------------+
4 rows in set (0.08 sec)

如您所见，我的表中有很多行，因此我不想使用子查询...

As you can see there are a lot of rows in my table, so for that reason I don't want to use subqueries...

编辑 3:

但是将 WHERE 添加到子查询中看起来更好...

But adding WHERE to the subquery it looks better...

mysql> DESCRIBE SELECT id_branch_channel, id_member, attendance, timestamp, id_member 
    -> FROM (select * from view_event_attendance where id_event = 782 order by timestamp desc) as whatever
    -> WHERE id_event = 782 
    -> GROUP BY id_event,id_member;
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+----------------------------------------------+
| id | select_type | table                       | type  | possible_keys   | key             | key_len | ref                                            | rows | Extra                                        |
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+----------------------------------------------+
|  1 | PRIMARY     | <derived2>                  | ALL   | NULL            | NULL            | NULL    | NULL                                           |   14 | Using where; Using temporary; Using filesort |
|  2 | DERIVED     | tbl_event                   | const | PRIMARY         | PRIMARY         | 4       |                                                |    1 | Using temporary; Using filesort              |
|  2 | DERIVED     | tbl_member_event            | ref   | id_event        | id_event        | 4       |                                                |   12 | Using index                                  |
|  2 | DERIVED     | tbl_member_event_attendance | ref   | id_event_member | id_event_member | 4       | video_staging.tbl_member_event.id_member_event |    1 | Using index                                  |
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+----------------------------------------------+
4 rows in set (0.01 sec)

如果我找不到任何不使用子查询的东西，我想我会选择这个作为答案...

If I can not find anything else not using subqueries, I think I'll choose this as the answer...

编辑 4

在看到答案中的评论后，我决定选择另一个作为答案.这是两个查询的 DESCRIBE，我认为最好的解决方案是显而易见的:

After seeing the comments in the answer, I've decided to select another as the answer. Here is the DESCRIBE for both queries, and I think it is obvious what is the best solution:

mysql> DESCRIBE SELECT 
    ->   id_branch_channel,
    ->   id_member, 
    ->   attendance, 
    ->   timestamp,
    ->   id_member
    -> FROM view_event_attendance AS t1
    -> WHERE id_event = 782
    -> AND timestamp = (SELECT MAX(timestamp)
    ->                  FROM view_event_attendance AS t2 
    ->                  WHERE t1.id_member = t2.id_member 
    ->                    AND t1.id_event = t2.id_event 
    ->                  GROUP BY id_event, id_member)
    -> OR timestamp IS NULL
    -> GROUP BY id_event, id_member;
+----+--------------------+-----------------------------+--------+--------------------+--------------------------+---------+------------------------------------------------+------+-----------------------------------------------------------+
| id | select_type        | table                       | type   | possible_keys      | key                      | key_len | ref                                            | rows | Extra                                                     |
+----+--------------------+-----------------------------+--------+--------------------+--------------------------+---------+------------------------------------------------+------+-----------------------------------------------------------+
|  1 | PRIMARY            | tbl_event                   | index  | PRIMARY            | id_member_branch_channel | 4       | NULL                                           |  208 | Using index; Using temporary; Using filesort              |
|  1 | PRIMARY            | tbl_member_event            | ref    | id_event           | id_event                 | 4       | video_staging.tbl_event.id_event               |   64 | Using index                                               |
|  1 | PRIMARY            | tbl_member_event_attendance | ref    | id_event_member    | id_event_member          | 4       | video_staging.tbl_member_event.id_member_event |    1 | Using where; Using index                                  |
|  2 | DEPENDENT SUBQUERY | tbl_event                   | eq_ref | PRIMARY            | PRIMARY                  | 4       | func                                           |    1 | Using where; Using index; Using temporary; Using filesort |
|  2 | DEPENDENT SUBQUERY | tbl_member_event            | eq_ref | id_event,id_member | id_event                 | 8       | video_staging.tbl_event.id_event,func          |    1 | Using where; Using index                                  |
|  2 | DEPENDENT SUBQUERY | tbl_member_event_attendance | ref    | id_event_member    | id_event_member          | 4       | video_staging.tbl_member_event.id_member_event |    1 | Using where; Using index                                  |
+----+--------------------+-----------------------------+--------+--------------------+--------------------------+---------+------------------------------------------------+------+-----------------------------------------------------------+
6 rows in set (0.00 sec)


mysql> DESCRIBE SELECT *
    -> FROM (SELECT id_branch_channel, id_member, attendance, timestamp, id_event 
    ->       FROM view_event_attendance 
    ->       WHERE id_event = 782 
    ->       ORDER BY timestamp desc
    ->      ) as whatever
    -> GROUP BY id_event,id_member;
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+---------------------------------+
| id | select_type | table                       | type  | possible_keys   | key             | key_len | ref                                            | rows | Extra                           |
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+---------------------------------+
|  1 | PRIMARY     | <derived2>                  | ALL   | NULL            | NULL            | NULL    | NULL                                           |   14 | Using temporary; Using filesort |
|  2 | DERIVED     | tbl_event                   | const | PRIMARY         | PRIMARY         | 4       |                                                |    1 | Using temporary; Using filesort |
|  2 | DERIVED     | tbl_member_event            | ref   | id_event        | id_event        | 4       |                                                |   12 | Using index                     |
|  2 | DERIVED     | tbl_member_event_attendance | ref   | id_event_member | id_event_member | 4       | video_staging.tbl_member_event.id_member_event |    1 | Using index                     |
+----+-------------+-----------------------------+-------+-----------------+-----------------+---------+------------------------------------------------+------+---------------------------------+
4 rows in set (0.00 sec)

推荐答案

使用 id_member 的简单组，但选择:

Use a simple group by id_member, but select:

substring(max(concat(from_unixtime(timestamp),attendance)) from 20) as attendance

这会将出勤附加到组中每一行的时间戳，以便能够使用 max() 选择所需的时间戳/出勤，然后仅提取出勤.

This attaches attendance to the timestamp for each row in a group, in order to be able to select the desired timestamp/attendance with max() and then extract just the attendance.

concat() 返回的是 19 个字符的格式化时间戳 (YYYY-mm-dd HH:MM:SS)，从字符 20 开始附加出勤；substring(... from 20) 仅从该组的(字符串式)最大出席人数中获取出席人数.您可以删除该组，只需

What concat() returns is 19 characters of formatted timestamp (YYYY-mm-dd HH:MM:SS) with the attendance appended starting at character 20; the substring(... from 20) gets just the attendance from the (stringwise) maximum one for the group. You can remove the group by and just

select concat(from_unixtime(timestamp),attendance), timestamp, attendance

更好地了解它如何使用 max 来获得合适的出勤率.

to get a better idea of how it uses max to get the right attendance.

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