MySQL 获取两个用户之间的对话

我有一个名为 private_messages 的 SQL 表，其中包含字段（id、from、to、message、stamp）。 标记字段对应于消息的日期

那么我需要什么查询：

1）获取两个用户之间的对话（按日期排序）？

我已经尝试过查询

(SELECT * FROM private_messages WHERE from=$my_id AND to=$other_id) 
UNION 
(SELECT * FROM private_messages WHERE from=$other_id AND to=$my_id) 
ORDER BY stamp
;

但不起作用...

2）获取我和其他用户之间的最后一条消息，每个消息都有一个不同的用户，按日期排序（例如构建一个像 Facebook 中的收件箱）？

1.)

SELECT  * 
FROM    private_messages a
WHERE   (a.from = $my_id AND a.to = $other_id) OR
        (a.from = $other_id AND a.to = $my_id)
ORDER   BY stamp DESC

2.)

SELECT  f.*
FROM
        (
            SELECT  *
            FROM    private_messages a
            WHERE  (LEAST(a.from, a.to), GREATEST(a.from, a.to), a.stamp) 
                    IN  (   
                            SELECT  LEAST(b.from, b.to) AS x, 
                                    GREATEST(b.from, b.to) AS y,
                                    MAX(b.stamp) AS msg_time
                            FROM    private_messages b
                            GROUP   BY x, y
                        )
        ) f
WHERE   $my_id IN (f.from, f.to)
ORDER   BY f.stamp DESC