MySQL的：使用UNION，MAX和GROUP BY

Question

奇怪的行为，我做了一个简单的表，用于存储信息：

+----+--------+----------+--------------+------+ 
| id | sender | receiver |  dt  | text | 
+----+--------+----------+--------------+------+ 
| 1 | a  | b  | ..19.26.00.. | msg1 | 
+----+--------+----------+--------------+------+ 
| 2 | c  | b  | ..19.26.02.. | msg2 | 
+----+--------+----------+--------------+------+ 
| 3 | b  | a  | ..19.26.03.. | msg3 | 
+----+--------+----------+--------------+------+ 

我想选择在交谈最近的消息。例如，对于B我想：

+--------------+--------------+------+ 
| conversation | MAX(maxdt) | text | 
+--------------+--------------+------+ 
| ab   | ..19.26.03.. | msg3 | 
+--------------+--------------+------+ 
| cb   | ..19.26.02.. | msg2 | 
+--------------+--------------+------+ 

所以我用这个查询：

SELECT conversation, MAX(maxdt), text FROM 
(SELECT CONCAT(sender, receiver) AS conversation, MAX(dt) AS maxdt, text 
FROM message 
WHERE receiver='b' GROUP BY conversation 
UNION 
SELECT CONCAT(receiver, sender) AS conversation, MAX(dt) AS maxdt, text 
FROM message 
WHERE sender='b' GROUP BY conversation) AS x 
GROUP BY conversation 

但结果是：

+--------------+--------------+------+ 
| conversation | MAX(maxdt) | text | 
+--------------+--------------+------+ 
| ab   | ..19.26.03.. | msg1 | 
+--------------+--------------+------+ 
| cb   | ..19.26.02.. | msg2 | 
+--------------+--------------+------+ 

因此，日期时间值是正确的，但文字来自错误的元组！

有什么建议吗？ SQL Fiddle

Answer 1

好吧我有一个可能的解决方案，工作SQL小提琴。试试这个：

SELECT CONCAT('b',other) AS conversation, text, dt AS maxdt FROM 
(
    SELECT IF(sender='b',receiver, sender) AS other, dt, text 
    FROM message 
    WHERE (receiver='b' OR sender='b') order by dt desc 
) 
AS TBL group by other 

如果你想让对话字段标准化，你可以CONCAT（'b'，other）作为对话。

Answer 2

 SELECT CONCAT(sender,receiver) AS conversation,maxdt,text 
    FROM 
     (SELECT max(dt) as maxdt FROM message WHERE sender = 'b' 
      UNION 
      SELECT max(dt) as maxdt FROM message WHERE receiver = 'b')T1, 
      message T2 
    WHERE T1.maxdt = T2.dt 
    AND (T2.receiver = 'b' OR T2.sender = 'b') 

sqlFiddle