Question

假设我有两张桌子，新闻和评论。

news (
  id,
  subject,
  body,
  posted
)

comments (
  id,
  parent,  // points to news.id
  message,
  name,
  posted
)

我想创建一个查询来抓取最新的x＃新闻项目以及每个新闻帖子的最新评论的名称和发布日期。

速度在选择子查询中的所有注释方面都很重要。

Answer 1

我刚刚意识到如果没有附加到新闻表的评论，查询就不会返回结果，这里是修复以及总帖子数量的添加列：

SELECT news.*, comments.name, comments.posted, (SELECT count(id) FROM comments WHERE comments.parent = news.id) AS numComments
FROM news
LEFT JOIN comments
ON news.id = comments.parent
AND comments.id = (SELECT max(id) FROM comments WHERE parent = news.id)

Answer 2

如果速度非常重要，为什么不创建一个包含最新注释的id和parent id的recent_comment表？每次在新闻帖子上发布评论时，请替换该新闻ID的最新评论ID。在新表的新闻ID列上创建一个索引，你的连接速度很快。

你的写入速度是读取速度，但不是很多。

Answer 3

假设已发布是唯一的时间戳，否则请选择唯一的自动编号

select c.id, c.parent, c.message, c.name, c.posted

  c.message, c.name, 

  c.posted -- same as comment_latest.recent

from comments c
join
(
    select parent, max(posted) as recent
    from comments
    group by parent
) as comment_latest
on c.parent = comment_latest.parent
and c.posted = comment_latest.recent

完成（显示新闻信息）：

select 

  n.id as news_id, n.subject, n.body, n.posted as news_posted_date

  c.id as comment_id, 
  c.message, c.name as commenter_name, c.posted as comment_posted_date

from comments c
join
(
    select r.parent, max(r.posted) as recent
    from comments r
    join 
    (
       select id from news order by id desc limit $last_x_news
    ) news l
    on r.parent = l.id
    group by r.parent
) as comment_latest
on c.parent = comment_latest.parent
and c.posted = comment_latest.recent

join news n on c.parent = n.id

<小时/> 注意：

上面的代码不是子查询，而是表派生查询。它比子查询更快。这是子查询（慢）：

select id, subject, body, posted as news_posted_date, (select id from comments where parent = news.id order by posted desc limit 1) as comment_id, (select message from comments where parent = news.id order by posted desc limit 1) as message, (select name from comments where parent = news.id order by posted desc limit 1) as name, (select posted from comments where parent = news.id order by posted desc limit 1) as comment_posted_date, from news

Answer 4

SELECT news.subject, news.body, comments.name, comments.posted
FROM news
INNER JOIN comments ON
(comments.parent = news.id)
WHERE comments.parent = news.id
AND comments.id = (SELECT MAX(id)
FROM comments
WHERE parent = news.id)
ORDER BY news.id

这会获取所有新闻项目，以及具有最高id值的相关评论，理论上这应该是最新的。

Answer 5

我的解决方案类似于 J ，但我认为他添加了一行不必要的内容：

SELECT news.*, comments.name, comments.posted FROM news INNER JOIN comments ON news.id = comments.parent WHERE comments.id = (SELECT max(id) FROM comments WHERE parent = news.id )

虽然不确定超大桌子上的速度。

Answer 6

鉴于我在其他答案的评论中所揭示的限制，我有一个新的想法，在实践中可能有或没有任何意义。

使用以下定义创建一个视图（或更合适的函数），名为recent_comments：

SELECT MAX(id), parent
FROM comments
GROUP BY parent

如果父列上有聚簇索引，这可能是一个相当快的查询，但即便如此，它仍然是一个瓶颈。

使用此功能，您需要获得答案的查询类似于

SELECT news.*, comments.*
FROM news
INNER JOIN recent_comments
ON news.id = recent_comments.parent
INNER JOIN comments
ON comments.id = recent_comments.id

另外还有对没有任何评论的新闻帖子的考虑。

Answer 7

我认为@Jan提供的解决方案是最好的。即创建“View”并使用SQL语句将其连接起来。

这肯定会减少提取数据的时间。我测试了它，它100％工作。

从MySQL中的另一个表中获取最新的行

7 个答案: