Question

我正试图加入一个所有主题所在的表格。我想要做的是列出所有论坛及其子类别，同时列出最新主题。

SELECT root.name AS root_name
   , subcat.name AS subcat_name
   , subcat.id AS subcat_id
   , subcat.description AS subcat_description
   , subcat.safe_url AS subcat_safe_url
   , topics.*
FROM forum_category AS root
LEFT JOIN forum_category AS subcat ON subcat.parent_id = root.id 
LEFT JOIN
(
   SELECT MAX(`last_post_time`) AS aaaa, last_post_time, topic_title
      , topic_id, forum_id
   FROM `forum_topics`
   WHERE 1
   GROUP BY forum_id
) AS topics ON topics.forum_id = subcat.id 
WHERE root.parent_id = 0 
ORDER BY root_name, subcat_name

但是现在我有点卡住:(，它如此接近，但目前它只在每个子论坛列出第一个主题，我需要最后一个但不知道如何。

Answer 1

您的子查询找到最后一篇文章的问题是，last_post_time，topic_title等没有理由属于具有MAX（last_post_time）的行。

考虑一下这个问题：

SELECT MAX(last_post_time), MIN(last_post_time), topic_title
FROM forum_topics
GROUP BY forum_id

返回哪个topic_title？那个帖子时间最长的那一行？那个帖子时间最少的那一行？这是不明确的 - MySQL只能从组中任意选择一个topic_title。在实践中，它从首先存储在组中的行中选择，并且在您的控制之外，取决于存储引擎实现等。

这是另一种设计，它找到了forum_topics行，其中没有其他forum_topics行存在且last_post_time更高：

SELECT root.name AS root_name
   , subcat.name AS subcat_name
   , subcat.id AS subcat_id
   , subcat.description AS subcat_description
   , subcat.safe_url AS subcat_safe_url
   , topics.*
FROM forum_category AS root
LEFT JOIN forum_category AS subcat ON subcat.parent_id = root.id 
LEFT JOIN forum_topics AS topics ON topics.forum_id = subcat.id
LEFT JOIN forum_topics AS t2 ON t2.forum_id = subcat.id 
  AND t2.last_post_time > topics.last_post_time
WHERE root.parent_id = 0 AND t2.forum_id IS NULL
ORDER BY root_name, subcat_name

Mysql LEFT JOIN - ＆gt;获取最新主题ID /主题标题

1 个答案: