mysql - 子选择还是加入？有没有更好的方法来编写这个 mysql 查询？

Question

我知道做某事总是有更好的方法，但我不确定该怎么做？优化此查询的最佳方法是什么？我应该使用联接、单独的查询等吗？我知道这不是一个复杂的查询……只是想扩展我的知识。

如有任何建议，我们将不胜感激!

SELECT
  community_threads.id AS thread_id,
  community_threads.title AS thread_title,
  community_threads.date AS thread_date,
  community_threads.author_id AS author_id,
  `user`.display_name AS author_name,
  `user`.organization AS author_organization,
  (SELECT date FROM community_replies replies WHERE replies.thread_id = community_threads.id ORDER BY date DESC LIMIT 1) AS reply_date,
  (SELECT   count(id) FROM community_replies replies WHERE replies.thread_id = community_threads.id ORDER BY date DESC LIMIT 1) AS total_replies
FROM
  community_threads
INNER JOIN `user` ON community_threads.author_id = `user`.id
WHERE
  category_id = '1'
ORDER BY
  reply_date DESC
LIMIT 0, 5

Answer 1

这可以通过针对子选择的 JOIN 进行改进，子选择获取每个 thread_id 的聚合 COUNT() 和聚合 MAX (日期)。派生表不应为每一行评估子选择，而应仅对整个查询评估一次，并与 community_threads 中的其余行连接。

SELECT
  community_threads.id AS thread_id,
  community_threads.title AS thread_title,
  community_threads.date AS thread_date,
  community_threads.author_id AS author_id,
  `user`.display_name AS author_name,
  `user`.organization AS author_organization,
  /* From the joined subqueries */
  maxdate.date AS reply_date,
  threadcount.num AS total_replies
FROM
  community_threads
  INNER JOIN `user` ON community_threads.author_id = `user`.id
  /* JOIN against subqueries to return MAX(date) (same as order by date DESC limit 1) and COUNT(*) from replies */
  /* number of replies per thread_id */
  INNER JOIN  (
    SELECT thread_id, COUNT(*) AS num FROM replies GROUP BY thread_id
  ) threadcount ON community_threads.id = threadcount.thread_id
  /* Most recent date per thread_id */
  INNER JOIN (
    SELECT thread_id, MAX(date) AS date FROM replies GROUP BY thread_id
  ) maxdate ON community_threads.id = maxdate.thread_id
WHERE
  category_id = '1'
ORDER BY
  reply_date DESC
LIMIT 0, 5

如果将 LIMIT 0, 5 放在 reply_date 子查询中，您可能会获得更好的性能。这只会在子查询中提取最近的 5 个，INNER JOIN 将丢弃所有来自 community_threads 的不匹配项。

/* I *think* this will work...*/
SELECT
  community_threads.id AS thread_id,
  community_threads.title AS thread_title,
  community_threads.date AS thread_date,
  community_threads.author_id AS author_id,
  `user`.display_name AS author_name,
  `user`.organization AS author_organization,
  /* From the joined subqueries */
  maxdate.date AS reply_date,
  threadcount.num AS total_replies
FROM
  community_threads
  INNER JOIN `user` ON community_threads.author_id = `user`.id
  INNER JOIN  (
    SELECT thread_id, COUNT(*) AS num FROM replies GROUP BY thread_id
  ) threadcount ON community_threads.id = threadcount.thread_id
  /* LIMIT in this subquery */
  INNER JOIN (
    SELECT thread_id, MAX(date) AS date FROM replies GROUP BY thread_id ORDER BY date DESC LIMIT 0, 5
  ) maxdate ON community_threads.id = maxdate.thread_id
WHERE
  category_id = '1'
ORDER BY
  reply_date DESC