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子选择还是联接？有没有更好的方法来编写这个mysql查询？

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子选择还是联接？有没有更好的方法来编写这个mysql查询？,mysql,sql,query-optimization,sql-optimization,Mysql,Sql,Query Optimization,Sql Optimization,我知道总有更好的方法做某事，但我不知道怎么做？优化此查询的最佳方法是什么？我应该使用连接、单独的查询等吗。。我知道这不是一个复杂的问题。。只是想扩大我的知识面 如有任何建议，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

我知道总有更好的方法做某事，但我不知道怎么做？优化此查询的最佳方法是什么？我应该使用连接、单独的查询等吗。。我知道这不是一个复杂的问题。。只是想扩大我的知识面

如有任何建议，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
这可以通过一个
JOIN
来改进，该JOIN与一个子选择相对应，该子选择获取聚合
COUNT（）
per
thread\u id
和聚合
MAX（date）
。不应该为每一行计算subselect，而应该为整个查询只计算一次派生表，并根据
community\u threads
中的其余行进行联接

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
限制0,5
放在
回复日期
子查询中，您可能会获得更好的性能。这将只提取子查询中最近的5个，而
内部联接将丢弃
社区线程中所有不匹配的线程


/* 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



这可以通过一个
JOIN
来改进，该JOIN与一个子选择相对应，该子选择获取聚合
COUNT（）
per
thread\u id
和聚合
MAX（date）
。不应该为每一行计算subselect，而应该为整个查询只计算一次派生表，并根据
community\u threads
中的其余行进行联接


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
限制0,5
放在
回复日期
子查询中，您可能会获得更好的性能。这将只提取子查询中最近的5个，而
内部联接将丢弃
社区线程中所有不匹配的线程


/* 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



据我所知，这看起来是一个很好的机会


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



希望这能有所帮助。
据我所知，这对一个小组来说似乎是一个很好的机会


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。
很有趣，谢谢。。以前从未想过使用最小/最大值。一旦我获得了声誉，我会投票的。很有趣，谢谢。。以前从未想过使用最小/最大值。一旦我获得了声誉，我就会投票支持。