SQL加入一個表和GROUP BY得到正確的行

Question

這裏是情況，我爲表forum_topics，forum_replies和users表。 其中_topics包含所有主題，_replies包含所有帖子，並且用戶包含所有用戶。

我想列出的主題爲：

（主題）_ __ _ __ _ __ _ __ _ __ _ __ _ __ _ __ _ __ _ __ _ __ _（最後回覆時間）

（通過用戶名創建主題）_ __ _ __ _ __ _ __ _ __ _ __（最後回覆方式）

現在，「主題」和「用戶名創建的主題」顯示得很好，但是，最後發佈的帖子和用戶的發佈時間這是錯的。

SQL：

SELECT 
forum_topics.id, 
forum_topics.category, 
forum_topics.subject, 
forum_topics.created AS topiccreate, 
forum_topics.createdby AS topiccreatedby, 
forum_replies.topic, 
forum_replies.created AS repliecreated, 
forum_replies.createdby AS repliecreatedby, 
usertopic.firstname AS topicfirstname, 
usertopic.lastname AS topiclastname, 
userreplie.firstname AS repliefirstname, 
userreplie.lastname AS replielastname, 
usertopic.id as topicid, 
userreplie.id 

FROM forum_topics 

JOIN forum_replies ON forum_replies.topic = forum_topics.id 
JOIN users usertopic ON forum_topics.createdby = usertopic.id 
JOIN users userreplie ON forum_replies.createdby = userreplie.id 

WHERE forum_topics.category = '1' 
GROUP BY forum_replies.topic 
ORDER BY forum_replies.created DESC 

我怎樣才能得到「最後回覆時間」和「最後回覆」，以顯示正確的嗎？我嘗試刪除Group By，然後它檢索所有帖子，但是我只想爲每個主題提供最新的後期數據。

至於現在使用GROUP BY時，它會檢索所有的主題只有一次（正確的），但是最後回覆時間最後回覆的顯示不正確的，因爲它似乎檢索數據爲每個第一篇文章話題。

希望你明白我的問題！ ：/

Answer 1

您還需要一個條件才能得到最新的答覆。這裏是你會怎麼做的join子句中：

FROM forum_topics 
JOIN forum_replies ON forum_replies.topic = forum_topics.id 
JOIN users usertopic ON forum_topics.createdby = usertopic.id 
JOIN users userreplie ON forum_replies.createdby = userreplie.id 
JOIN (select topic, max(created) as maxcreated 
     from forum_replies 
     group by topic 
    ) frmax on frmax.topic = forum_replies.topic and 
       frmax.maxcreated = forum_replies.created; 

我不認爲你將需要group by這樣做了。

Answer 2

嘗試在您的SQL查詢中刪除這兩個組並按順序查詢並檢查。