在 mysql 中使用 group by 仅选择最后一个值 [英] Select only last value using group by at mysql
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问题描述
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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