php - MySQL 选择多行/组多/顺序

Question

我正在构建一个聊天系统，用户 A 可以在其中与用户 B 讨论主题 X 或 Y。

我的表如下所示:

Message ID | message | sender | receiver | topic
1            Helo      1        2           X
2            Lorem     2        1           X
3            Foo       1        2           Y
4            Bar       2        1           X
5            Test      2        1           Y
6            Hello2    1        3           X

所以我想获取每个用户和主题的用户 1 最后 6 个对话的列表。目前我得到了

SELECT messageid, sender,topic, receiver AS friend FROM table WHERE sender = 1 || receiver = 1 GROUP BY topic,friend ORDER BY id DESC LIMIT 0,6

但这不会选择最后消息。因为之后我创建了这些条目的列表，并且不想在顶部显示最后的对话...

在此示例中，我需要一个如下所示的列表:

 Message ID | sender | topic| friend
 6            1         X      3
 5            2         Y      1
 4            2         X      1

通过以下选择就可以了! select * from (从选项卡中选择 Message_ID、发送者、主题、接收者 AS friend ，其中发送者 = 1 || 接收者 = 1 按 Message_ID desc 排序) temp 按主题分组 order by Message_ID desc

有效!

Answer 1

如果您想要给定用户的最后一条消息 id 的 ID:

select (case when sender = 1 then receiver else sender end) as other,
       topic, max(messageid)
from messages
where sender = 1 or receiver = 1
group by (case when sender = 1 then receiver else sender end) ;

然后您可以使用它来获取所有信息:

select m.*
from messages m join
     (select (case when sender = 1 then receiver else sender end) as other,
             topic, max(messageid) as messageid
      from messages
      where sender = 1 or receiver = 1
      group by (case when sender = 1 then receiver else sender end) 
     ) mm
     on mm.messageid = m.messageid;