MySQL 获取最后一条消息 (VarChar)

Question

我正在尝试从某个线程获取最后一条消息，但似乎无法正确查询。我似乎没有收到某个主题的最后一条消息。

简介

我使用了 3 个表 -- message，message_users，message_threads

所有消息都属于某个主题（有点像论坛和帖子的运作方式？）用户可以将多个用户添加到某个线程，这样多个用户就可以在同一个线程中互相发消息了。

数据库结构

messages table
    id  |   t_id  |  u_id  |  message  |  time
--------+---------+--------+-----------+--------
      1 |    1    |   1    |  hello!   | timestamp
      2 |    1    |   2    |  hi!      | timestamp
      3 |    2    |   1    |  testing  | timestamp


message_threads
   id |  author |  subject  
------+---------+-----------
    1 |    1    | Subject 1  
    2 |    3    | Subject 2 
    3 |    2    | Subject 3


message_users
   id |   t_id  |  u_id  |  read  | nonotice
------+---------+--------+--------+----------
    1 |    1    |   1    |   0    |   0
    2 |    1    |   2    |   0    |   0
    3 |    2    |   1    |   1    |   0

查询

这是我正在使用的查询，它几乎可以工作，只是它没有获得最后的 ID，也没有获得最后的消息，有什么想法吗？（是的，我已经查看并搜索过有关此主题的先前问题，但无法以某种方式根据答案修复查询）

SELECT
  m2.message,
  mt.id AS thread_id,
  max(m.id) AS message_id, 
  max(m.time) AS time, 
  mt.subject
FROM message_users AS mu
LEFT JOIN message_threads AS mt ON mt.id = mu.t_id
LEFT JOIN message AS m  ON m.t_id = mt.id
INNER JOIN (
  select message as messages_text 
  from message group by t_id
) AS m2 ON m2.id = m.id
WHERE mu.u_id = 1 AND mu.read = 0 AND mu.nonotice = 0
GROUP BY mt.id
ORDER BY m.id DESC

Answer 1

SELECT
  m.t_id,
  m.message,
  m.id AS max_message_id,
  m.ts AS max_message_ts,
  t.subject
FROM messages AS m
JOIN message_threads AS t ON m.t_id = t.id
JOIN (
  SELECT t_id, max(id) max_id FROM messages GROUP BY 1
) AS m2 ON m.id = m2.max_id
JOIN message_users AS u ON u.t_id = t.id AND u.u_id = 1 AND u.`read` = 0 AND u.nonotice = 0

如果上面 SQL 变慢，您可以执行以下任一操作：

1 - 将 where 添加到内部联接 SQL 中以使结果更小，即：

SELECT
  m.t_id,
  m.message,
  m.id AS max_message_id,
  m.ts AS max_message_ts,
  m2.subject
FROM messages AS m
JOIN (
  SELECT t.id, max(m.id) AS max_id, t.subject
  FROM messages AS m
  JOIN message_threads AS t ON m.t_id = t.id
  JOIN message_users AS u ON u.t_id = t.id 
     AND u.u_id = 1 AND u.`read` = 0 AND u.nonotice = 0
  GROUP BY 1
) AS m2 ON m.id = m2.max_id

2 - 重新设计 message_thread 添加计算列：(last_message_id, last_updated) 等

PS - 尽量避免使用 MySQL 关键字作为您的列名称

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