Question

我们可以在没有子查询的情况下进行此查询吗？

SELECT login, post_n,

(SELECT SUM(vote) FROM votes WHERE votes.post_n=posts.post_n)AS votes, 
(SELECT COUNT(comments.post_n) FROM comments WHERE comments.post_n=posts.post_n)AS comments_count 

FROM users, posts 
WHERE posts.id=users.id AND (visibility=2 OR visibility=3) 
ORDER BY date DESC LIMIT 0, 15

表：

用户：id，login

帖子：post_n，id，visibility

投票：post_n，投票

id - 它是用户ID，用户是主表。

Answer 1

是的，有可能：

SELECT login, post_n,

SUM(vote) as votes,

FROM users 
JOIN posts using(id)
LEFT JOIN votes using(post_n)
WHERE visibility=2 OR visibility=3 
GROUP BY login, post_n

然后压扁结果：

select * from 
(
    SELECT login, post_n,

    SUM(vote) as votes,

    FROM users 
    LEFT JOIN posts using(id)
    LEFT JOIN votes using(post_n)
    WHERE visibility=2 OR visibility=3 
    GROUP BY login, post_n
) as votes_count

然后加入评论：

select votes_count.login, votes_count.post_n, votes_count.votes, 

    COUNT(comments.post_n) as comments_count

from 
(
    SELECT login, post_n,

    SUM(vote) as votes,

    FROM users 
    LEFT JOIN posts using(id)
    LEFT JOIN votes using(post_n)
    WHERE visibility=2 OR visibility=3 
    GROUP BY login, post_n
) as votes_count
LEFT JOIN comments using(post_n)
GROUP BY votes_count.login, votes_count.post_n
ORDER BY date DESC LIMIT 0, 15

Answer 2

您可以在“用户”表格中存储投票金额和帖子计数，并通过触发器或“更新”查询进行更新。

Answer 3

我在使用join或在哪里合并表时测试了变体和测试变体。

没有子查询变量更慢，0.0208秒，而mysql在投票表中仅使用271行，但是当我使用连接时，它使用整行。这里没有子查询变体：

SELECT res.*, COUNT(*) FROM
(
  SELECT login, posts.post_n, SUM(vote)AS votes
  FROM users, posts, votes
  WHERE users.id=posts.id AND posts.post_n=votes.post_n AND visibility=3
  GROUP BY posts.post_n
)AS res, comments
WHERE comments.post_n=res.post_n
GROUP BY res.post_n
ORDER BY date DESC
LIMIT 0, 15

子查询varian仅执行0.0027秒，它没有缓存，但在所有测试中都使用了索引。

P.S。对不起我的英文

MySQL子查询

3 个答案: