MySQL - Order by on multiple left join

Question

所以我有表 ForumRooms、ForumTopics 和 ForumPosts，它们很容易解释。

目前，我正在更改对主论坛房间页面的查询，以便能够获取存储在 ForumPosts 中的最后发帖时间和用户。

表结构

论坛室

|  id  |  title  |
|----------------|
|  1   | Room 1  |
|  2   | Room 2  |

论坛主题

|  id  |  title    |  forum_room_id  |
|------------------|-----------------|
|  1   | Thread 1  |  1              |
|  2   | Thread 2  |  2              |
|  3   | Thread 3  |  1              |

论坛帖子

|  id  |  content  |  forum_thread_id |  post_time  |  user_id  |
|------------------|------------------|-------------|-----------|
|  1   | A post 1  |  1               |  15/02/2015 |  1        |
|  2   | A post 2  |  2               |  16/02/2015 |  2        |
|  3   | A post 3  |  1               |  17/02/2015 |  1        |
|  4   | A post 4  |  1               |  18/02/2015 |  2        |

目标

我正在尝试编写 1 个查询以允许以下格式化输出:

输出

|  ForumRoom  |  Thread Count  |  Post Count  |  Last Post Time  |  Last Post UserID  |
|-------------|----------------|--------------|------------------|--------------------|
|  Room 1     |  2             |  3           |  16/02/2015      |  2                 |
|  Room 2     |  1             |  1           |  18/02/2015      |  2                 |

问题

目前，除了Last Post Time 和Last Post UserID 之外，我可以在 1 个查询中获取所有内容。

由于 ForumPosts.id 上的 ORDER BY，下面的查询应该给出我需要的，但我认为 GROUP BY 阻止了它从按要求工作。

SELECT ForumRooms.title, COUNT(DISTINCT ForumThreads.id), COUNT(ForumPosts.id), ForumPosts.post_time, ForumPosts.user_id
FROM ForumRooms
LEFT JOIN ForumThreads ON ForumRooms.id = ForumThreads.forum_room_id
LEFT JOIN ForumPosts ON ForumThreads.id = ForumPosts.forum_thread_id
GROUP BY ForumRooms.id
ORDER BY ForumPosts.id DESC

如何调整我的查询以获取最后两个缺失的数据？

Answer 1

这可能是使用 substring()/group_concat() 技巧的好地方:

SELECT ForumRooms.title, COUNT(DISTINCT ForumThreads.id), COUNT(ForumPosts.id), 
       MAX(ForumPosts.post_time),
       SUBSTRING_INDEX(GROUP_CONCAT(ForumPosts.user_id ORDER BY ForumPosts.post_time DESC), ',', 1)
FROM ForumRooms LEFT JOIN
     ForumThreads
     ON ForumRooms.id = ForumThreads.forum_room_id LEFT JOIN
     ForumPosts
     ON ForumThreads.id = ForumPosts.forum_thread_id
GROUP BY ForumRooms.id
ORDER BY ForumPosts.id DESC