检索评论的回复数

问题描述

我有以下PostgreSQL表来存储对不同帖子的评论：

评论表：

commentID | parentID | postID | content | author | created

我现在想使用给定postID的单个查询来检索帖子的所有评论以及每个评论的回复数。如果parentID不为null且等于其父对象的commentID，则注释就是答复。

我尝试了以下类似的方法，在该表上我自己加入了表并寻找parentID = commentID的匹配项，但是我无法使其正常运行，希望能有所帮助：）

SELECT comments.commentID as commentID,comments.parentID as parentID,comments.postID as postID,comments.content as content,comments.author as author,comments.created as created,COALESCE(c1.numReplies,0) as numReplies
FROM comments c0
LEFT JOIN (
    SELECT parentID,COUNT(*) FILTER (WHERE postID = :givenPostID) as numReplies as numReplies
    FROM comments
) c1 on c0.commentID = c1.parentID
WHERE c0.postID = :givenPostID;

解决方法

这看起来像是子查询或横向联接的好地方：

select c.*,c1.no_replies
from comments c
cross join lateral (
    select count(*) no_replies
    from comments c1
    where c1.parentid = c.commentid
) c1 
where c.postid = :givenpostid

旁注-您要编写的查询可能是：

SELECT c0.*,COALESCE(c1.numReplies,0) as numReplies
FROM comments c0
LEFT JOIN (
    SELECT parentID,COUNT(*) as numReplies
    FROM comments
    GROUP BY parentID
) c1 on c0.commentID = c1.parentID
WHERE c0.postID = :givenPostID

子查询略有不同：首先，它需要一个GROUP BY子句才是有效的SQL；而且，不需要条件计数。