Question

我知道总会有更好的办法，但我不知道怎么办？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。我知道这不是一个复杂的查询..只是试图扩展我的知识。

任何建议都将不胜感激！

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

可以使用JOIN针对子选项进行改进，该子选择获得每COUNT()聚合thread_id和聚合MAX(date)。不是评估每一行的子选择，而是应该对整个查询仅计算一次派生表，并将其与community_threads中的其余行连接起来。

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，则可能会获得更好的性能。这只会拉出子查询中最近的5个，INNER JOIN会丢弃community_threads中不匹配的所有内容。

/* 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

Answer 2

据我所知，这似乎是一个小组的好机会。

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,
    MAX(replies.date) AS reply_date,
    COUNT(replies.id) AS total_replies
FROM community_threads
INNER JOIN `user` ON community_threads.author_id = `user`.id
INNER JOIN community_replies AS replies ON replies.thread_id = community_threads.id
WHERE category_id = 1
GROUP BY thread_id, thread_title, thread_date, author_id, author_name, author_organization)
ORDER BY reply_date DESC
LIMIT 0, 5

希望这有帮助。

SubSelect或Joins？有没有更好的方法来编写这个mysql查询？

2 个答案: