MySQL复杂的多表计数答案

【问题标题】：MySQL complex multi table countMySQL复杂的多表计数
【发布时间】：2014-12-28 05:38:06
【问题描述】：

在MySQL中，我试图获取forum_posts中的行数，其中id等于forum_posts_threads中的post_id列，其中thread_id列等于forum_threads_forums中的thread_id列，其中forum_id 列匹配某个值。举例说明：

forum_forums

id  name
1   forum1
2   forum2

forum_threads

id  title
1   thread1
2   thread2

forum_threads_forums

thread_id  forum_id
1          1
1          2
2          2

forum_posts

id  title
1   post1
2   post2

forum_posts_threads

post_id  thread_id
1        1
2        1
2        2

然后我正在执行一个获取所有论坛的查询。我想要的是统计每个论坛的帖子数量。

所以，它需要返回如下内容：

id  name    post_count
1   forum1  2
2   forum2  3

我已经有以下查询：

SELECT
    forum_forums.id,
    forum_forums.name,
    forum_forums.description,
    COUNT(forum_threads_forums.thread_id) AS thread_count,
    forum_categories.id AS category_id,
    forum_categories.name AS category_name,
    forum_categories.description AS category_description
FROM
    forum_forums
LEFT OUTER JOIN
    forum_threads_forums
ON
    forum_forums.id=forum_threads_forums.forum_id
INNER JOIN
    forum_forums_categories
ON
    forum_forums.id=forum_forums_categories.forum_id
INNER JOIN
    forum_categories
ON
    forum_forums_categories.category_id=forum_categories.id
GROUP BY
    forum_forums.id

该查询已经能够计算线程的数量（并执行一些其他操作），但我不确定如何计算帖子，因为它需要检查两个单独表中的条件。

因此，如果有人可以就如何调整我的查询提供一些建议，那就太好了。

提前致谢！

【问题讨论】：

如果将第一段中的“table 1”、“table 2”等替换为实际的表名，您的问题会清晰很多。
@Tom 我会的，很抱歉有任何混淆

标签： php mysql pdo aggregate-functions

【解决方案1】：

我认为您正在尝试计算 forum_threads_forums 中带有线程的帖子数量。有几种方法可以做到这一点。

最简单的方法可能就是加入forum_posts_threads 表并在post_id 上执行COUNT DISTINCT。由于笛卡尔坐标，您还必须更改您的 thread_count 以执行 COUNT DISTINCT。

SELECT
    forum_forums.id,
    forum_forums.name,
    forum_forums.description,
    COUNT(DISTINCT forum_threads_forums.thread_id) AS thread_count,
    COUNT(DISTINCT forum_posts_threads.post_id) AS post_count,
    forum_categories.id AS category_id,
    forum_categories.name AS category_name,
    forum_categories.description AS category_description
FROM
    forum_forums
LEFT OUTER JOIN
    forum_threads_forums
ON
    forum_forums.id=forum_threads_forums.forum_id
LEFT OUTER JOIN
    forum_posts_threads
ON
    forum_threads_forums.thread_id=forum_posts_threads.thread_id
INNER JOIN
    forum_forums_categories
ON
    forum_forums.id=forum_forums_categories.forum_id
INNER JOIN
    forum_categories
ON
    forum_forums_categories.category_id=forum_categories.id
GROUP BY
    forum_forums.id

如果您需要将某些条件应用于 forum_posts 表，您可以在 SELECT 子句中添加一个子查询。像这样的东西（在下面的WHERE 子句中添加）。它将引用您的主要 FROM 子句中的 forum_threads_forums.thread_id。

(SELECT COUNT(DISTINCT forum_posts.post_id ) post_count
 FROM   forum_posts
        JOIN forum_post_threads ON forum_posts.post_id = forum_post_threads.post_id
 WHERE  forum_post_threads.thread_id = forum_threads_forums.thread_id
        [ADD IN YOUR ADDITIONAL WHERE CONDITIONS]
) as post_count

【讨论】：