如何在 MySQL 中的 GROUP BY 之前使用 ORDER BY

Question

我有两个表（消息和用户）

Messages 表有以下字段 {message, senderid, receiverid} 用户表有以下字段 {userid, name, email}

我想运行一个查询，以便仅获取发送给特定用户的最后一条消息。 这是查询

SELECT m.message, u.name, u.email 
from users as u, messages as m 
where m.sender = u.userid 
and m.receiver = '1' 
group by m.sender desc;

但是，这不会返回我想要的，因为它为每条发送的消息选择第一条消息，而不是最后一条消息。一个完美的解决方案应该是

SELECT m.message, u.name, u.email 
from users as u, messages as m 
where m.sender = u.userid 
and m.receiver = '1' 
order by m.sentdate desc 
group by m.sender desc;

但我无法在 GROUP BY 之前使用 ORDER 以便从最后一条消息到第一条消息排序，然后按用户分组。

请帮忙

Answer 1

试试这个

SELECT
    x.message,
    x.name,
    x.email
FROM (
    SELECT
        m.message,
        u.name,
        u.email 
    FROM users AS u, messages AS m
    WHERE m.sender = u.userid AND m.receiver = '1'
    ORDER BY m.sentdate DESC
) x
GROUP BY sender, email, message;

Answer 2

如果你只想拉最后一条消息，你可以试试-

SELECT m.message, u.name, u.email 
  FROM users as u
  JOIN messages as m ON m.sender = u.userid
  JOIN (SELECT sender, MAX(sentdate) sentdate
          FROM messages
         GROUP BY sender) m2 ON m.sender = m2.sender
                             AND m.sentdate = m2.sentdate
 WHERE m.receiver = '1';

Answer 3

您可以使用以下查询来获取每个用户的最后 N 条消息。

SELECT * FROM messages M1 WHERE M1.senddate 
   IN 
   (SELECT TOP 4 senddate FROM messages 
      WHERE sender=M1.sender 
      ORDER BY senddate DESC)
   AND receiver = 1;

在子查询中，根据您的用例将顶部从 4 修改为 N。如果您只想要每个用户的最后 1 条消息，请将其更改为 1