MySQL：选择线程和帖子，按最新活动排序[重复]答案

【问题标题】：MySQL: Select threads and posts, order by latest activity [duplicate]MySQL：选择线程和帖子，按最新活动排序[重复]
【发布时间】：2013-12-15 00:05:30
【问题描述】：

我想获取forum 的threads 列表，按最新活动排序：

创建线程
已创建帖子

期望的输出：

Thread name  | Timestamp (Thread) | Timestamp (latest post)
-----------------------------------------------------------
Thread A     | 1                  | 10                 <- old thread, newest post (newer than newest thread)
Thread C     | 3                  | -                  <- newest thread, no post
Thread B     | 2                  | 5

编辑/可能的解决方案：

SELECT
    t.*,
    IFNULL
    (         
        (
            SELECT p.timestamp 
              FROM posts p 
             WHERE p.thread_id = t.id 
             ORDER BY p.timestamp DESC LIMIT 1
        ),
        t.timestamp
    ) AS sorting 
FROM 
    threads t
WHERE t.forum_id = 1
ORDER BY sorting DESC

有人有性能建议吗？谢谢大家！

【问题讨论】：

样本数据，期望的结果。
关于该主题的快速 google 出现了以下链接：stackoverflow.com/questions/13181966/… 该帖子中似乎有两个选项作为答案提交。我不是 SQL 专家，所以我不能确定你会如何处理这个问题，你可以从他们构建查询的方式中收集一些信息。

标签： mysql sql database select join

【解决方案1】：

SELECT t.*, IFNULL(MAX(p.`timestamp`), t.`timestamp`) AS pts FROM `threads` t, `posts` p WHERE p.`thread_id` = t.`id` AND t.`forum_id`=1 ORDER BY pts DESC

【讨论】：

【解决方案2】：

试试这个：

SELECT threads.name
  , threads.timestamp
  , latest_posts.latest_timestamp
  , GREATEST(threads.timestamp,COALESCE(latest_posts.latest_timestamp,threads.timestamp)) AS sorting
FROM threads
LEFT JOIN 
    (SELECT thread_id, MAX(timestamp) as latest_timestamp
    FROM posts
    GROUP BY thread_id) as latest_posts
  ON threads.id = latest_posts.thread_id
ORDER BY sorting

【讨论】：