如何由一個列組，但選擇一個值

Question

這是我的消息

我想組由receiver_id數據庫，這是我的SQL

SELECT * FROM messages WHERE sender_id=2 
ORDER BY created_at DESC 
GROUP BY receiver_id 
HAVING COUNT(receiver_id)>=1 

它的工作，但它總是顯示主題Bok和created_at=2013-08-19 20:49:22 我想顯示最新created_at和主題，在這個例子中2013-08-20 14:29:41

這是我的輸出

Answer 1

SELECT 
     m1.* 
FROM 
     messages m1 
LEFT JOIN 
     messages m2 
ON 
    (m1.receiver_id = m2.receiver_id 
    AND m1.created_at < m2.created_at) 
WHERE 
    m2.created_id IS NULL; 

Answer 2

GroupWise的最大幫助我http://dev.mysql.com/doc/refman/5.0/en/example-maximum-column-group-row.html 所以我的查詢看起來像

...WHERE created_at=(SELECT MAX(created_at) as created_at FROM mg_messages t2 WHERE t.receiver_id=t2.receiver_id)...