MySQL 选择 GROUP BY 命令

Question

我有一个mysql语句

SELECT * 
FROM tbl_messages 
WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
GROUP BY from_user_id 
ORDER BY date_sent DESC

它产生了正确的结果，但它们的顺序不正确。

分组效果很好，但组中显示的记录是输入数据库的第一条记录，但我希望在每个组中显示最新记录。

有没有办法让每个组显示最新的记录？

2011-12-19 12:16:25 This is the first message
2011-12-19 12:18:20 This is the second message
2011-12-19 12:43:04 This is the third message

该组显示“这是第一条消息”，我想要“这是第三条消息”，因为这是最新的记录/消息。

干杯

Answer 1

这可能有效(但不能保证):

SELECT * 
FROM
  ( SELECT *
    FROM tbl_messages 
    WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
    ORDER BY date_sent DESC
  ) tmp
GROUP BY from_user_id 
ORDER BY date_sent DESC

---

这应该有效:

SELECT t.* 
FROM 
    tbl_messages AS t
  JOIN
    ( SELECT from_user_id 
           , MAX(date_sent) AS max_date_sent
      FROM tbl_messages 
      WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
      GROUP BY from_user_id 
    ) AS tg
    ON  (tg.from_user_id, tg.max_date_sent) = (t.from_user_id, t.date_sent)
ORDER BY t.date_sent DESC