Question

所以我正在开发一个拥有大量多对数据库关系的网站，因为我对查询关系数据库还有点新意，所以我有点挣扎。所以我有几个表，我试图从中获取相关数据。我不会进入整个数据库，因为它非常大，我试图获得所有特定用户帖子的评论数量和帖子的喜欢数量，以及我的方式关于它是这样使用LEFT JOIN

  SELECT Post.idPosts, Post.Title, Post.Date_poste,
  COUNT(Post_has_Comments.Post_idPost), 
  COUNT(Post_has_Likes.Post_idStories)
  FROM Post
  LEFT JOIN Post_has_Comments ON Post.idPost = S    
Post_has_Comments.Post_idStories
LEFT JOIN Post_has_Likes ON Post.idPost = Post_has_Likes.Post_idStories
WHERE Post.idUsers  =  1

但我遇到的问题是，如果没有评论或没有喜欢这将返回错误，此外如果有类似或评论它将返回两个字段中的最高数字，例如，如果有对帖子有3条评论，1条也会在类似字段中返回3，因为它正在计算它返回的行数我猜。所以我的问题是如何实际获得喜欢和评论的真实数量并放入该字段，并且如果没有错误而不是错误则返回0？

Answer 1

SELECT Post.idPosts, Post.Title, Post.Date_poste,
    coalesce(cc.Count, 0) as CommentCount,
    coalesce(lc.Count, 0) as LikeCount
FROM Post p 
left outer join(
    select Post_idPost, count(*) as Count
    from Post_has_Comments
    group by Post_idPost
) cc on p.idPost = cc.Post_idPost
left outer join (
    select Post_idStories, count(*) as Count
    from Post_has_Likes
    group by Post_idStories
) lc on p.idPost = lc.Post_idStories
WHERE p.idUsers = 1

Answer 2

您可以使用嵌套查询返回计数：

，而不是使用count（）

 SELECT Post.idPosts, Post.Title, Post.Date_poste,
  select COUNT(Post_idPost) from Post_has_Comments where Post_has_Comments.Post_idPost = Post.idPosts, 
  ...

Answer 3

您必须重命名具有count（）的列的结果，如此

SELECT Post.idPosts, Post.Title, Post.Date_poste,
  COUNT(Post_has_Comments.Post_idPost) AS comments_count, 
  COUNT(Post_has_Likes.Post_idStories) AS likes_count
  FROM Post
  LEFT JOIN Post_has_Comments ON Post.idPost = S    
Post_has_Comments.Post_idStories
LEFT JOIN Post_has_Likes ON Post.idPost = Post_has_Likes.Post_idStories
WHERE Post.idUsers  =  1

具有多个COUNT（）的多个LEFT JOINS

3 个答案: