得到GROUP BY的語句

Question

最新的文本輸入我有一個數據庫，在那裏我把郵件會話

table "conversations" 
ID SUBJECT 
1  meeting on Friday 

table "conversations_mails" 
ID CONVERSATION_ID TEXT        CREATED_ON 
1  1     "What about a meeting on Friday?" 2012-08-05 10:00:00 
2  1     "that's a good idea!"    2012-08-10 15:00:00 

現在，我想顯示的對話概述頁面，展示了最新的答覆的文本被截斷的版本。例如

"Meeting on Friday" 
That's a good ... 

我嘗試通過GROUP BY實現此目的。但我得到的是表格中的第一個答覆（「星期五會議怎麼樣」），而不是最後一個「這是一個好主意」。

這是我的SQL語句：

SELECT *, MAX(conversations_mails.created_on) As conversation_last_reply, 
    MAX(conversations_mails.id) AS maxId 
FROM conversations 
LEFT JOIN conversations_mails ON conversations_mails.conversation_id = conversations.id 
GROUP BY conversations.id 
ORDER BY conversation_last_reply DESC 

我知道如何通過MAX（）獲得最高的ID，但對於相應的文本輸入？

謝謝！

Answer 1

嘗試這 -

SELECT *, MAX(conversations_mails.created_on) As conversation_last_reply, 
    MAX(conversations_mails.id) AS maxId 
FROM conversations 
LEFT JOIN conversations_mails ON conversations_mails.conversation_id = conversations.id 
WHERE conversations_mails.id = (select max(conversations_mails.id) from conversations_mails where conversations_mails.conversation_id = conversations.id) 
GROUP BY conversations.id 
ORDER BY conversation_last_reply DESC 

Answer 2

有兩個選項：使用子查詢（如geeky_bat建議），或添加另一JOIN過濾掉舊的行。下面是如何與另一個JOIN做到這一點：

SELECT 
    cm.created_on AS conversation_last_reply, 
    cm.id AS maxId, 
    cm.conversation_text 
FROM conversations c 
    LEFT OUTER JOIN conversations_mails cm 
    ON cm.conversation_id = c.id 
    LEFT OUTER JOIN conversations_mails cm_newer 
    ON cm_newer.conversation_id = c.id 
    AND cm_newer.created_on > cm.created_on 
WHERE cm_newer.created_on IS NULL 
GROUP BY c.id 
ORDER BY cm.created_on DESC 

http://sqlfiddle.com/#!2/5dfea/18