通过会话线程中的最新响应对消息进行分组

Question

我需要在用户之间使用简单的内部消息系统.

我的桌子:

+--------------+         +---------------------+
|   messages   |         |        users        |
+----+---------+         +---------------------+
| id | message |         | id | username | ... 
+----+---------+         +---------------------+

+------------------------------------------------------------------------------+
|                                 users_messages                               |
+------------------------------------------------------------------------------+
| id | from_usr_id | to_usr_id | msg_id | thread_id | read | sent_at | read_at |
+------------------------------------------------------------------------------+

INT 'thread_id' 表示对话线程,用于对消息进行分组.

BOOLEAN 'read' 表示用户是否打开/查看了消息.

我想按消息分组'thread_id',按顺序排列,'sent_at'我可以通过线程向用户显示他的最新消息.我还要计算每个线程中的消息.

我想为特定的用户ID获取类似的内容:

+----------------------------------------------------------------------------
| last_messages_by_conversation
+----------------------------------------------------------------------------
| message | from_username | sent_at | count_thread_msgs | count_unread_msg |
+----------------------------------------------------------------------------

TEXT 'message' 是具体的最新消息 'thread_id'

VARCHAR 'from_username'并DATETIME 'sent_at'与最新消息有关.

INT 'count_thread_msgs'并且INT 'count_unread_msg'与线程相关,表示线程中的消息总数和未读消息数.

每行代表一个线程/对话(分组依据'thread_id'),显示该'sent_at'特定线程的最后一条消息(按排序).

Answer 1

您正在寻找groupwise最大值,可以通过首先对users_messages表进行分组thread_id并选择MAX(sent_at),然后将结果重新连接到users_messages表上以查找该最大记录的其他字段来找到.

我发现这NATURAL JOIN是一个非常方便的捷径:

SELECT   messages.message,
         users.username AS from_username,
         t.sent_at,
         t.count_thread_msgs,
         t.count_unread_msg
FROM     users_messages NATURAL JOIN (
  SELECT   thread_id,
           to_usr_id,
           MAX(sent_at)  AS sent_at,
           COUNT(*)      AS count_thread_msgs,
           SUM(NOT read) AS count_unread_msg
  FROM     users_messages
  WHERE    to_usr_id = ?
  GROUP BY thread_id
) t JOIN messages ON messages.id = users_messages.msg_id
    JOIN users    ON users.id    = users_messages.from_usr_id