多个连接和总和的问题

Question

如果有以下三个PostgreSQL表：

发布表：

postid | title | author | created

投票表：

postid | username | vote

如果用户对帖子投了赞成票，则 vote 等于 1，如果用户没有投票，则为 0，如果用户对帖子投反对票，则为 -1。

评论表：

commentID | parentID | postID | content | author | created

如果评论不是回复，则 parentID 为 null。

我想现在收到每个帖子的标题、作者、创建日期、所有投票的总和以及 当前登录用户的投票和 cmets 的数量。 我已经对用户的投票有疑问，并询问了here，有人帮我得到了以下查询：

SELECT post.postID as postID, post.title as title, post.author as author,
       post.created as created,
       COALESCE(sum(votes.vote), 0) as voteCount,
       COALESCE(sum(votes.vote) FILTER (WHERE votes.username = :username), 0) as userVote
       FROM post 
       LEFT JOIN votes ON post.postID = votes.postID 
       GROUP BY post.postID 
       ORDER BY voteCount DESC

现在我尝试了另一个 LEFT JOIN 来获取这样的 cmets 数量：

COUNT(DISTINCT comments) FILTER (WHERE comments.parentID IS NULL) as numComments
LEFT JOIN comments on post.postID = comments.postID

但是，虽然 cmets 的数量有效，但每个帖子的投票数是错误的，因为 由于另一个连接，这些行似乎多次出现，产生了错误的总和，我在想办法解决这个问题时遇到了一些麻烦。

我已经尝试将 cmets 的数量作为子查询获取，以便它独立于 没有成功的票数。 任何进一步的帮助将不胜感激！ :-)

Answer 1

分别计算值。 joins 导致笛卡尔积。这是相关子查询或横向连接有帮助的地方：

SELECT p.*, v.*, c.*
FROM post p CROSS JOIN LATERAL
     (SELECT SUM(v.vote) as voteCount,
             SUM(v.vote) FILTER (WHERE v.username = :username), 0) as userVote
      FROM votes v
      WHERE p.postID = v.postID 
     ) v CROSS JOIN LATERAL
     (SELECT SUM(c.vote) as commentCount,
             SUM(c.vote) FILTER (WHERE c.username = :username), 0) as userVote
      FROM comments c
      WHERE p.postID = c.postID 
     ) c
ORDER BY voteCount DESC;

Answer 2

您通常会在加入之前预先聚合子查询，如下所示：

SELECT p.*
    COALESCE(v.voteCount, 0) as voteCount,
    COALESCE(v.userVote, 0) as userVote,
    COALESCE(c.numComments, 0) as numComments
FROM post p
LEFT JOIN (
    SELECT postID, 
        SUM(vote) as voteCount, 
        SUM(vote) FILTER (WHERE username = :username) userVote
    FROM votes 
    GROUP BY postID
) v ON v.postID = p.postID 
LEFT JOIN (
    SELECT postID, count(*) numComments
    FROM comments
    WHERE parentID IS NULL
    GROUP BY postID
) c ON c.postID = p.postID
ORDER BY voteCount DESC