Question

好的，SO挖掘的时间，我仍然找到了一个解决方案 - 一个IMO相当明显 - 的任务。我有posts，我想在每个帖子（最新的）中查询最多 5 comments。

所以基本上是这样的：

SELECT p.id, p.title, c.id, c.text
FROM posts p
LEFT JOIN comments c ON p.id = c.postId LIMIT 5

（伪，不起作用）

如何限制加入？

Answer 1

SELECT  *
FROM    posts p
LEFT JOIN
        comments c
ON      c.post_id = p.id
        AND c.id >=
        COALESCE(
                (
                SELECT  ci.id
                FROM    comments ci
                WHERE   ci.post_id = p.id
                ORDER BY
                        ci.post_id DESC, ci.id DESC -- You need both fields here for MySQL to pick the right index
                LIMIT   4, 1
                ), 0
                )

在comments (post_id)或comments (post_id, id)（如果comments为MyISAM）上创建索引，以便快速开展此工作。

Answer 2

这看起来像[greatest-n-per-group]问题。该链接指向此站点上的其他标记问题。我会首先获取您的所有帖子/评论，然后您可以将其限制为每个帖子的最近5个，如下所示：

SELECT p1.*, c1.*
FROM posts p1
LEFT JOIN comments c1 ON c1.post_id = p1.id
WHERE(
   SELECT COUNT(*)
   FROM posts p2
   LEFT JOIN comments c2 ON c2.post_id = p2.id
   WHERE c2.post_id = c1.post_id AND c2.commentDate >= c1.commentDate
) <= 5;

这是关于该主题的另一个reference。

Answer 3

您可以使用变量：

SELECT pid, title, cid, text
FROM (
  SELECT p.id AS pid, p.title, c.id AS cid, c.text,
         @row_number:= IF(@pid = p.id,
                          IF (@pid:=p.id, @row_number+1, @row_number+1),
                          IF (@pid:=p.id, 1, 1)) AS rn
  FROM posts p
  CROSS JOIN (SELECT @row_number := 0, @pid := 0) AS vars
  LEFT JOIN comments c ON p.id = c.postId 
  ORDER BY p.id ) t  <-- add comments ordering field here
WHERE t.rn <= 5

Demo here

如何限制加入？

3 个答案: