Question

如下设置有效。但是，我想要实现的是计算BlogComment BlogComment.BlogPostId = BlogPost.BlogPostId中的行数。

有人有意见吗？

SELECT * FROM BlogPost
INNER JOIN BlogUser
ON BlogPost.BlogUserId = BlogUser.BlogUserId
INNER JOIN Category
ON BlogPost.CategoryId = Category.CategoryId
INNER JOIN BlogComment
ON BlogPost.BlogPostId = BlogComment.BlogPostId
Order By BlogPost.BlogPostId Desc

Answer 1

每个博客帖子你想要多少评论？那应该是

SELECT blogpostid, 
       COUNT(*) as TotalComments 
FROM   blogpost p 
       JOIN blogcomment c 
         ON p.blogpostid = c.blogpostid 
GROUP  BY blogpostid;

Answer 2

如果您想要所有数据加上计数（而不仅仅是计数），您可以在from子句中创建内联查询来执行此操作。

SELECT *, 
       COALESCE(c.count_post_id,0) as count_post_id
FROM   blogpost 
       INNER JOIN bloguser 
         ON blogpost.bloguserid = bloguser.bloguserid 
       INNER JOIN category 
         ON blogpost.categoryid = category.categoryid 
       LEFT JOIN (SELECT blogpostid, 
                          COUNT(blogpostid) count_post_id
                   FROM   blogcomment
                   GROUP BY blogpostid) c
         ON c.blogpostid = blogpost.blogpostid 
ORDER  BY blogpost.blogpostid DESC

某些RDMS提供了其他选项，例如使用CTE或Cross Apply

Answer 3

SELECT COUNT(*) 
FROM   blogpost a 
WHERE  EXISTS (SELECT * 
               FROM   blogcomment 
               WHERE  blogcomment.blogpostid = a.blogpostid)

Answer 4

尝试：

SELECT BlogPost.*, BlogUser.*, Category.*, BlogComment.*, COUNT(BlogComment.BlogCommentId) FROM BlogPost
INNER JOIN BlogUser
ON BlogPost.BlogUserId = BlogUser.BlogUserId
INNER JOIN Category
ON BlogPost.CategoryId = Category.CategoryId
INNER JOIN BlogComment
ON BlogPost.BlogPostId = BlogComment.BlogPostId
GROUP BY BlogPost.BlogPostId Order By BlogPost.BlogPostId Desc

SQL内部连接和计数

4 个答案: