如何使用 LEFT JOIN 并根据计数将结果限制为一行

Question

假设我有 3 tables:

Table 'post'

id	title
0	titleA
1	titleB
2	titleC

Table 'comment'

id	id_post	text
0	1	blaa
1	3	blbb
2	5	blcc

Table 'like'

id	id_comment	vote
0	1	+1
1	5	-1
2	5	+1

我需要获得 post 条评论最多的列表。

查询应该对每个post的每个评论的table“喜欢”（非真实姓名）的投票列（+1，-1 ...）求和，保留higest 加到对应的post.

例如：

post_id	post_title	comment_id	comment_text	like_vote
0	titleA	12	blaaaa	51
1	titleB	25	blabbb	98
2	titleC	63	blaccc	14

$statement = $this->pdo->prepare('SELECT SQL_NO_CACHE p.id AS post_id, p.title AS post_title, c.id AS comment_id, c.text AS comment_text, IFNULL(like.like_vote, 0) AS like_vote FROM post p
    LEFT JOIN  (SELECT * FROM comment) c ON p.id = c.id_post
    LEFT JOIN  (SELECT id_comment, IFNULL(SUM(vote), 0) AS like_vote, FROM like GROUP BY id_comment ORDER BY like_vote DESC) like ON c.id = like.id_comment 
    ORDER BY p.id ASC
    ');

我得到的结果是，当 post 有 2 条评论时，我得到 2 post 每条评论和计数如下：

post_id: 0, comment_id: 1, like_vote: 5
post_id: 0, comment_id: 2, like_vote: 7
post_id: 1, comment_id: 3, like_vote: 10
post_id: 1, comment_id: 4, like_vote: 3
...

每条评论都得到正确的赞数，但我不知道如何只保留最高评论并避免多行 post 具有相同的 id，像这样：

post_id: 0, comment_id: 2, like_vote: 7
post_id: 1, comment_id: 3, like_vote: 10

如果你能帮助我构建查询那就太好了，我只是找不到它...

编辑：（table 赞的名称只是一个例子，以保持清楚）

Answer 1

你可以写这个查询，你会得到预期的结果。

我只是将 table like 重命名为 comment_like 因为 like 是预定义的词，所以使用预定义的词不是好的做法。

    SELECT p.id AS post_id, p.title AS post_title, c.id AS comment_id, c.text AS comment_text, IFNULL(comment_like.like_vote, 0) AS like_vote FROM post p 
LEFT JOIN comment c ON p.id = c.id_post 
LEFT JOIN (SELECT id_comment, IFNULL(SUM(vote), 0) AS like_vote FROM comment_like GROUP BY id_comment ORDER BY like_vote DESC) comment_like ON c.id = comment_like.id_comment ORDER BY p.id ASC;

Answer 2

一种方法是使用 CTE。使用 ROW_NUMBER() OVER(...) 按 post 和最大票数对结果进行排序和排名。然后拿最大的那个，即where votes_rank = 1

post_id	comment_id	like_votes	votes_rank
0	1	5	1
0	2	7	2
1	4	3	1
1	3	10	2

SQL (MySQL 8.x)

注意 - 使用 table 名称 comment_likes 以避免必须转义关键字 like

WITH cte AS (
   SELECT p.id AS post_id
          , c.id AS comment_id
          , l.like_votes
          , ROW_NUMBER() OVER(
              PARTITION BY p.id
              ORDER BY l.like_votes DESC
          ) AS votes_rank
   FROM   post p 
             LEFT JOIN comment c ON c.id_post = p.id
             LEFT JOIN (
                 SELECT id_comment, SUM(vote) AS like_votes
                 FROM   comment_likes
                 GROUP BY id_comment
             )
             l ON l.id_comment = c.id
)
SELECT * 
FROM   cte
WHERE  votes_rank = 1
AND    like_votes IS NOT NULL
ORDER BY post_id, like_votes

结果：

post_id	comment_id	like_votes	votes_rank
0	2	7	1
1	3	10	1

db<>fiddle here

更新 2022-04-01

MySQL 5.x 不支持 window 函数，因此您必须求助于子查询。这样的事情应该会产生类似的结果

 SELECT   p.id AS post_id
          , ( SELECT  c.id AS comment_id
               FROM   comment c INNER JOIN comment_likes l ON l.id_comment = c.id
               WHERE  c.id_post = p.id
               GROUP BY c.id
               ORDER BY SUM(vote) DESC
               LIMIT 1
           ) AS comment_id
           , ( SELECT SUM(l.vote) 
               FROM   comment c INNER JOIN comment_likes l ON l.id_comment = c.id
               WHERE  p.id = c.id_post
               GROUP BY c.id
               ORDER BY SUM(l.vote) DESC
               LIMIT 1
           ) AS total_like_votes
 FROM   post p 
 HAVING total_like_votes IS NOT NULL

结果：

post_id	comment_id	total_like_votes
0	2	7
1	3	10

db<>fiddle here

如何使用 LEFT JOIN 并根据计数将结果限制为一行

How to use LEFT JOIN and limit result to one row based on a count

mysql

sql

mysql-5.7