使用UNION和SORT的MySql查询

实际上，我正在尝试创建一个像FB（Messages）这样的对话界面，并为此使用一个sql查询来获取所有已经与用户交谈过的人。 我需要按降序排列与他通话的用户的id， 例如，如果A与B和C聊天，那么B和C将是该查询的结果，B将首先出现，因为A最近与B聊天

我的“消息”表结构是：

这是一个例子：

这是我的错误查询，需要修复：

---

$uid将为我提供要获取列表的用户的变量（这里是A）。

您似乎得到了主要用户和他们交谈的对象，而不管哪一个是主要用户。也不太确定MySQL将如何计算出您使用DISTINCT时的排序时间，DISTINCT将删除一些记录及其时间

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，如果给同一个人的多条消息都共享同一个最新时间，这可能会有点混淆。如果有可能，可按如下方式处理：-

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

---

那么什么不起作用呢？它不会以排序的方式给出结果，我的意思是B应该排在第一位，然后是C。拥有最新消息的用户将在结果的顶部出现，然后是第二位最新消息所有者，依此类推。老兄，你是摇滚明星！谢谢。是否也可以在结果中列出

状态

？（读/未读最新消息）这是可能的，但要复杂一点。我们将有一个快速播放

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