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将联接查询重写为子查询,缺少行?

subquery mysql

cabrerahector · 技术社区 · 6 年前

我有以下返回这些行的MySQL查询:

SELECT p.ID AS id, p.post_title AS title, p.post_author AS uid, SUM(v.pageviews) AS pageviews, IFNULL(c.comment_count, 0) AS comment_count 
FROM `views` v 
LEFT JOIN `posts` p ON v.postid = p.ID 
LEFT JOIN (SELECT comment_post_ID, COUNT(comment_post_ID) AS comment_count FROM `comments` WHERE comment_date_gmt > DATE_SUB('2018-08-16 18:34:46', INTERVAL 29 DAY) AND comment_approved = 1 GROUP BY comment_post_ID) c ON p.ID = c.comment_post_ID 
WHERE 1 = 1 AND p.post_type IN('post') AND v.view_datetime > DATE_SUB('2018-08-16 18:34:46', INTERVAL 29 DAY) AND p.post_password = '' AND p.post_status = 'publish' GROUP BY v.postid ORDER BY pageviews DESC LIMIT 5 OFFSET 0;

我目前正在尝试使用子查询而不是联接来重写它,以检查性能是否更好。

下面的查询返回相同的结果(除了 comment_count 列,因为我们只查询两个表而不是三个表):

SELECT p.ID AS id, p.post_title AS title, p.post_author AS uid, v.pageviews AS pageviews 
FROM `posts` p,
    (SELECT SUM(pageviews) AS pageviews, postid FROM `views` WHERE view_datetime > DATE_SUB( '2018-08-16 18:34:46', INTERVAL 29 DAY) GROUP BY postid) v
WHERE 1 = 1 AND p.ID = v.postid AND p.post_type IN('post') AND p.post_password = '' AND p.post_status = 'publish' GROUP BY v.postid ORDER BY pageviews DESC LIMIT 5 OFFSET 0

但是,如果我加上 comments 表中,没有注释的帖子会被自动排除在外(比如ID) 2956 与原始查询相比,返回不同的结果:

SELECT p.ID AS id, p.post_title AS title, p.post_author AS uid, v.pageviews AS pageviews, IFNULL(c.comment_count, 0) AS comment_count 
FROM `posts` p,
    (SELECT SUM(pageviews) AS pageviews, postid FROM `views` WHERE view_datetime > DATE_SUB( '2018-08-16 18:34:46', INTERVAL 29 DAY) GROUP BY postid) v,
    (SELECT COUNT(comment_post_ID) AS comment_count, comment_post_ID FROM `comments` WHERE comment_date_gmt > DATE_SUB('2018-08-16 18:34:46', INTERVAL 29 DAY) AND comment_approved = 1 GROUP BY comment_post_ID) c 
WHERE 1 = 1 AND p.ID = v.postid AND v.postid = c.comment_post_ID AND p.post_type IN('post') AND p.post_password = '' AND p.post_status = 'publish' GROUP BY v.postid ORDER BY pageviews DESC LIMIT 5 OFFSET 0

我对这些东西还不是很有经验(你可以告诉我),所以任何帮助/提示来理解正在发生的事情都将非常感激。

1 回复 | 直到 6 年前

Nick SamSmith1986 6 年前

SELECT id, post_title AS title, post_author AS uid, 
    (SELECT SUM(pageviews) 
     FROM views v 
     WHERE v.postid = posts.id AND 
           v.view_datetime > DATE_SUB('2018-08-16 18:34:46', INTERVAL 29 DAY)
    ) AS pageviews,
    (SELECT COUNT(comment_post_ID) 
     FROM comments c 
     WHERE c.comment_post_ID = posts.id AND
           c.comment_date_gmt > DATE_SUB('2018-08-16 18:34:46', INTERVAL 29 DAY) AND 
           c.comment_approved = 1
    ) AS comment_count
FROM posts
WHERE p.post_type IN('post') AND p.post_password = '' AND p.post_status = 'publish'
ORDER BY pageviews DESC 
LIMIT 5 OFFSET 0;