如何由一个列组，但选择一个值

Question

这是我的消息

我想组由receiver_id数据库，这是我的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，但它总是显示主题Bok和created_at=2013-08-19 20:49:22 我想显示最新created_at和主题，在这个例子中2013-08-20 14:29:41

这是我的输出

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的最大帮助我http://dev.mysql.com/doc/refman/5.0/en/example-maximum-column-group-row.html 所以我的查询看起来像

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