mysql - 每个有条件的 parent 的平均值

Question

我有父 posts 表和子 votes 与 posts.id 和 votes.post_id 关系。我想计算每个帖子的平均评分，但只计算最近给定的 50 票。我知道如何为所有选票做到这一点:

SELECT T1.`title`, (
    SELECT AVG(`vote`)
    FROM `votes`
    WHERE `votes`.`post_id` = T1.`id`
) AS `average`
FROM `posts` T1
GROUP BY T1.`id`

我知道这可能与子查询有关:

SELECT T1.`title`, (
    SELECT AVG(`vote`)
    FROM (
        SELECT `vote` FROM `votes`
        WHERE `votes`.`post_id` = T1.`id`
        ORDER BY `votes`.`id` DESC
        LIMIT 10
    ) AS T2
) AS `average`
FROM `posts` T1
GROUP BY T1.`id`

但出现错误:查询错误 (1054):“where 子句”中的未知列“T1.id”。 T1 别名在子子查询中不可访问。有什么想法吗？

http://sqlfiddle.com/#!9/fb9341/2

Answer 1

好的，您想要选择每个帖子最近 50 行中的行。 Stack Overflow 上有很多针对此类查询的答案，大部分在 greatest-n-per-group 下。或 limit-per-group标签。示例:How to SELECT the newest four items per category?

一旦你编写了该查询，你就可以像你已经知道如何编写的那样放入一个子查询，以获得每个帖子的 AVG(vote)。

---

回复你的评论:

这就是我的意思:

SELECT T1.title, AVG(V.vote) AS avg_vote 
FROM posts T1 
JOIN (
   SELECT v1.id, v1.post_id, v1.vote
   FROM votes v1
   LEFT OUTER JOIN votes v2 ON v1.post_id = v2.post_id and v1.id < v2.id
   GROUP BY v1.id
   HAVING COUNT(*) < 10
) AS V ON T1.id = V.post_id
GROUP BY T1.id;

给定 SQLFiddle 中数据的输出:

+---------+----------+
| title   | avg_vote |
+---------+----------+
| Title 1 |   5.4000 |
| Title 2 |   4.2000 |
+---------+----------+

为了帮助子查询中的 JOIN，您应该在列(post_id、id)上的 votes 上建立索引。

---

这是另一个不需要唯一列的解决方案:

SELECT T1.title, AVG(V.vote) AS avg_vote
FROM posts T1
JOIN (
    SELECT *
    FROM (
        SELECT v.*, @r := IF(@p = post_id, @r+1, 1) AS rownum, @p := post_id
        FROM (SELECT @p:=null, @r:=0) AS _init
        CROSS JOIN votes v
        ORDER BY v.post_id, v.id DESC
    ) AS t
    WHERE t.rownum <= 10
) AS V ON T1.id = V.post_id
GROUP BY T1.id;

输出与之前的查询相同。