PHP和Mysql-两个表之间的左外部联接

Question

我有两个名为“事件”和“主题”的表，每个表可以有很多注释。

我需要做的是列出所有事件和主题以及每行的注释量。 我设法返回了所有主题，效果很好，但是我不知道如何将事件表添加到MySql中。 注释和事件表字段在下面列出。 有人可以帮我这个查询吗？

大事记：

• ID
• 事件名称

评论：

• post_id <-事件或主题表的相关ID

• table <-该行所属的表，因此主题或事件

   SELECT t.id, t.title, c.created_at, IF(ISNULL(c.allComments), 0, c.allComments) AS totalComments FROM topics AS t LEFT OUTER JOIN ( SELECT created_at, post_id, COUNT(*) AS allComments FROM comments GROUP BY post_id ) AS c ON c.post_id = t.id ORDER BY tc.created_at DESC, c.allComments DESC 

Answer 1

听起来事件和主题应该在同一张桌子上。

不过，我认为我们可以使用UNION做到这一点。 我希望事件和主题具有相同的列？ （或者至少是同样重要的？）

(SELECT c.table as event_or_topic, e.*, count(C.table), MAX(C.created_at) as latest_c
FROM events E LEFT JOIN comments C on (C.post_id = E.id)
WHERE C.table = 'Events' 
GROUP BY C.post_id)
UNION
(SELECT c.table as event_or_topic, t.id*, count(C.table), MAX(C.created_at) as latest_c
FROM topics T LEFT JOIN comments C on (C.post_id = E.id)
WHERE C.table = 'Topics' 
GROUP BY C.post_id)
ORDER BY latest_c

注意，ORDER BY适用于整个UNION，而不适用于单个SELECT。

使用LEFT JOIN应该允许那些没有注释的行仍然显示。 我认为问题在于我们选择的部分内容依赖于注释（即-C.table，最后注释的排序等）。 计数应该可以-如果没有评论，则为零。

您可能需要稍微更改SELECT部分​​。 我想显示C.table，以便您知道行是主题还是事件，但恐怕它可能会增加计数。 除了计数之外，您还需要其他注释吗？ 您在查询中使用了post_id和table以外的其他列，而这些列在问题中被忽略了。

您仍然有不知道它们是什么的列，例如Comment的zoneTable

Answer 2

尝试这个：

SELECT t.id, t.title, c.created_at, COUNT(c.allComments) AS totalComments FROM topics AS t LEFT JOIN comments c ON t.id=c.post_id GROUP BY t.id ORDER BY tc.created_at DESC, c.allComments DESC

Answer 3

如果我理解您的问题，那么您有3个表格：

-事件

-主题

-评论

如果是这样，则应提取所有数据：

SELECT *
FROM events,topics
LEFT JOIN comments ON post_ID = ID
ORDER BY date DESC

希望我走对了！

W.

Answer 4

我已经工作了。 如果有人知道更好，更有效的方法，请告诉我：

(SELECT t.id, t.title, tc.dateCreated AS commentDate, 
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments,
t.LastActive as dateChanged
    FROM Events AS t
    LEFT OUTER JOIN (
          SELECT MAX(created_at) AS dateCreated, post_id,
          COUNT(*) AS allComments 
          FROM comments
          GROUP BY post_id
          ) AS tc ON tc.post_id = t.id)
UNION
(SELECT t.id, t.title, tc.dateCreated AS commentDate, 
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments,
t.LastActive as dateChanged 
    FROM topics AS t
    LEFT OUTER JOIN (
          SELECT MAX(created_at) AS dateCreated, post_id,
          COUNT(*) AS allComments 
          FROM comments
          GROUP BY post_id
          ) AS tc ON tc.post_id = t.id)
ORDER BY commentDate DESC, dateChanged DESC, totalComments DESC