sql - 如何从 SQL 中的同一个表的各种连接中获取计数？

Question

我有两个表，博客和帖子，它们之间具有预期的关系。帖子可以发布或草稿。我想选择所有包含已发布帖子和草稿帖子的博客。我该怎么做？

我试过这个查询:

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

但我最终得到了 published_post_count 和 draft_post_count 相同的数字，但两者都是错误的。

然后我绝望了，尝试:

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得到了相同的错误结果。

执行此操作的正确方法是什么？

谢谢。

Answer 1

如果您只对 COUNT 感兴趣，并且您还在 JOIN 表中。因此，必须有一个或多个 post 已被 published 和一个或多个 draft。所以也许这会对你有所帮助:

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