Question

我有两个表，博客和帖子，它们之间有预期的关系。帖子可以发布或草稿。我想选择所有已发布和草稿帖子的博客。我该怎么办？

我尝试了这个查询：

SELECT blogs.*,
       COUNT(published_posts.*) AS published_post_count,
       COUNT(draft_posts.*) AS draft_post_count
FROM blogs
JOIN posts AS published_posts ON published_posts.blog_id=blogs.id AND 
                                 published_posts.state = "published"
JOIN posts AS draft_posts ON draft_posts.blog_id=blogs.id AND 
                             draft_posts.state = "draft"
GROUP BY blogs.id

但是对于published_post_count和draft_post_count，我的结果都是相同的，但两者都错了。

然后我变得绝望和特里：

SELECT blogs.*,
       COUNT(published_posts.*) AS published_post_count,
       COUNT(draft_posts.*) AS draft_post_count
FROM blogs
JOIN (SELECT * FROM posts WHERE posts.state="published") AS published_posts
     ON published_posts.blog_id=blogs.id 
JOIN (SELECT * FROM posts WHERE posts.state="draft") AS draft_posts
     ON draft_posts.blog_id=blogs.id
GROUP BY blogs.id

但在这两种情况下我都有相同的错误结果。

这样做的正确方法是什么？

感谢。

Answer 1

如果您只对COUNT感兴趣，并且您也JOIN了表格。因此，必须有一个或多个post已published且一个或多个draft。所以这可能会对你有所帮助：

SELECT blogs.*,
       (
            SELECT COUNT(*)
            FROM posts
            WHERE posts.blog_id=blogs.id
            AND posts.state = "published"
       ) AS published_post_count,
       (
            SELECT COUNT(*)
            FROM posts
            WHERE posts.blog_id=blogs.id
            AND posts.state = "draft"
       ) AS draft_post_count,
FROM blogs

Answer 2

尝试：

GROUP BY blogs.id,published_posts.[theKeyOfThisTable],draft_posts.[theKeyOfThisTable]

因为您应该在“group by”子句中包含“select”的列

Answer 3

您可能会计算不同的状态并加入博客：

SELECT blogs.*, p.published_post_count, p.draft_post_count
FROM blogs
JOIN 
(
  SELECT posts.blog_id,
         SUM(case when posts.state = 'published' then 1 end) 
             AS published_post_count,
         SUM(case when posts.state = 'draft' then 1 end) 
             AS draft_post_count
    FROM posts
   GROUP BY blog_id
) p
ON p.blog_id=blogs.id

如何从SQL中的各个连接计数到同一个表？

3 个答案: