MySQL查询与UNION和SORT

Question

实际上，我正在尝试创建一个类似于FB（Messages）的对话界面，为此，使用一个SQL查询来获取所有已经与用户交谈过的人。 我需要按降序与之交谈的用户的ID，例如如果A已与B和C聊天。那么B AND C将是该查询的结果，而B将首先出现是因为A最近与B聊天。

我的“邮件”表结构为： http : //www.softnuke.com/me/files/DB.png

这是FB示例： http : //www.softnuke.com/me/files/msg.png

这是我不正确的查询，需要解决：

SELECT DISTINCT(`mates`)FROM(
    SELECT `time` AS `time`,`from_id` AS `mates` 
     FROM `messages` AS T WHERE (`from_id`=$uid OR `to_id`=$uid) 

    UNION

    SELECT `time` AS `time`,`to_id` AS `mates`
    FROM `messages` AS T WHERE (`from_id`=$uid OR `to_id`=$uid) 

    ) AS T 
    WHERE `mates`!='$uid'
    ORDER BY `time`

$ uid将给我要获取列表的用户的变量（此处为A）。

Answer 1

您似乎正在成为主要人物以及他们正在与之交谈的人，而不论哪个人是主要人物。 同样也不十分清楚，当您使用DISTINCT时，MySQL将如何计算出时间来排序商品，这会随着时间删除某些记录。

您可以通过以下方式获得最长时间和订单：-

SELECT `mates`, MAX(`time`) AS LatestConv
FROM(
    SELECT `time` AS `time`,`from_id` AS `mates` 
     FROM `messages` AS T WHERE `to_id`=$uid

    UNION

    SELECT `time` AS `time`,`to_id` AS `mates`
    FROM `messages` AS T WHERE `from_id`=$uid 

    ) AS T 
GROUP BY `mates`
ORDER BY LatestConv

要获取最新消息的状态，请执行以下操作：-

SELECT a.mates, a.LatestConv, IFNULL(b.Status, c.Status)
FROM
(
    SELECT mates, MAX(`time`) AS LatestConv
    FROM(
        SELECT `time` AS `time`, from_id AS mates 
        FROM messages AS T 
        WHERE to_id = $uid
        UNION
        SELECT `time` AS `time`, to_id AS mates
        FROM messages AS T 
        WHERE from_id = $uid 
        ) AS T 
    GROUP BY `mates`
) a
LEFT OUTER JOIN messages b
ON a.mates = b.from_id AND a.LatestConv = b.`time` AND b.to_id = $uid
LEFT OUTER JOIN messages c
ON a.mates = c.to_id AND a.LatestConv = c.`time` AND c.from_id = $uid
ORDER BY LatestConv

请注意，如果有多个发给同一个人的消息都共享相同的最新时间，这可能会使您感到困惑。 如果可能的话，可以采取以下措施：

SELECT a.mates, a.LatestConv, MAX(IFNULL(b.Status, c.Status))
FROM
(
    SELECT mates, MAX(`time`) AS LatestConv
    FROM(
        SELECT `time` AS `time`, from_id AS mates 
        FROM messages AS T 
        WHERE to_id = $uid
        UNION
        SELECT `time` AS `time`, to_id AS mates
        FROM messages AS T 
        WHERE from_id = $uid 
        ) AS T 
    GROUP BY `mates`
) a
LEFT OUTER JOIN messages b
ON a.mates = b.from_id AND a.LatestConv = b.`time` AND b.to_id = $uid
LEFT OUTER JOIN messages c
ON a.mates = c.to_id AND a.LatestConv = c.`time` AND c.from_id = $uid
GROUP BY a.mates, a.LatestConv
ORDER BY LatestConv