mysql - SQL:选择最新主题和最新帖子，按论坛分组，按最新帖子排序

Question

我正在尝试获取

• 最新主题(id、主题、时间戳、author_id)和
• 最新帖子(id、thread_id、时间戳、author_id)
• 每个论坛 (id, name)
• 按最新帖子排序，独立于话题的创建日期。

为什么？

我希望能够显示如下详细信息:

"The latest Answer of forum $forum_id was given on Question $thread_id. Here it is: $post_id"

---

SELECT  f.id AS forum_id,
        f.name AS forum_name,
        t.id AS thread_id,
        t.topic AS thread_topic,
        t.ts AS thread_timestamp,
        p.id AS post_id,
        p.content AS post_content,
        p.ts AS post_timestamp

 FROM   forums f,
        threads t,
        posts p

WHERE   f.id = t.forum_id 
  AND   t.id = p.thread_id

GROUP BY f.id
ORDER BY p.ts

---

任何建议，如何更改 SQL 以获得尽可能高的性能所需的结果？我试图避免子查询，但我很开放!

提前致谢!

Answer 1

由于 MySQL 不支持窗口函数，我认为没有子查询就没有办法做到这一点:

SELECT  f.id AS forum_id,
    f.name AS forum_name,
    t.id AS thread_id,
    t.topic AS thread_topic,
    t.ts AS thread_timestamp,
    p.id AS post_id,
    p.content AS post_content,
    p.ts AS post_timestamp

FROM   forums f
JOIN (SELECT t2.forum_id, max(p2.ts) as ts
      FROM posts p2
      JOIN threads t2 ON p2.thread_id = t2.id
      GROUP BY t2.forum_id) max_p ON f.id = max_p.forum_id
JOIN   posts p ON max_p.ts = p.ts
JOIN   threads t ON f.id = t.forum_id AND p.thread_id = t.id
ORDER BY p.ts

自然地，缓存最新的结果可以让你在没有调用 MAX() 的性能损失的情况下执行此操作，但是使用正确的索引，这应该不是什么大问题......

更新

包含没有帖子的话题和没有话题的论坛的最简洁的方法是使用 LEFT JOIN 而不是 INNER JOIN:

SELECT  f.id AS forum_id,
    f.name AS forum_name,
    t.id AS thread_id,
    t.topic AS thread_topic,
    t.ts AS thread_timestamp,
    p.id AS post_id,
    p.content AS post_content,
    p.ts AS post_timestamp

FROM   forums f
LEFT JOIN (SELECT t2.forum_id, max(COALESCE(p2.ts, t2.ts)) as ts, COUNT(p2.ts) as post_count
      FROM threads t2 
      LEFT JOIN posts p2 ON p2.thread_id = t2.id
      GROUP BY t2.forum_id) max_p ON f.id = max_p.forum_id
LEFT JOIN   posts p ON max_p.ts = p.ts
LEFT JOIN   threads t ON f.id = t.forum_id AND (max_p.post_count = 0 OR p.thread_id = t.id)
ORDER BY p.ts