Question

以下是聊天消息和用户的表格结构。

我希望来自用户的所有对话的最新1条消息，例如用户2

ChatMsgs
========
+--------+-----------+----------+-----------+
| ChatID | FKFromUID |  FKToUID |  Message  |
+--------+-----------+----------+-----------+
|      1 |         3 |        2 |    Hello1 |
|      2 |         2 |        1 |    Hello2 |
|      3 |         2 |        3 |    Hello3 |
|      4 |         3 |        2 |    Hi1    |
|      5 |         2 |        3 |    Hi2    |
|      6 |         3 |        2 |    Hi3    |
+--------+-----------+----------+-----------+

Users
======
+--------+----------+
| UserID | Fullname |
+--------+----------+
|      1 |    User1 |
|      2 |    User2 |
|      3 |    User3 |
+--------+----------+

我想将结果过滤为特定用户，例如用户2

+--------+----------+---------+
| UserID | Fullname | Message |
+--------+----------+---------+
|      3 |  User3   | Hi3     |
|      1 |  User1   | Hello2  |
+--------+----------+---------+

正如您在结果集中看到的那样，User2的信息/列没有，但是对于其他用户，但只有那些在FKFromUID或FKToUID中存在User2的记录。

这是我创建的查询，但是如果用户退出它，则会提供额外的记录。

select U.UserID
     , FullName
     , Message 
  from ChatMsgs C 
  JOIN 
     ( select max(ChatID) chatID 
         from ChatMsgs 
        group 
           by FKToUID
         , FKFromUID
     ) CC 
    ON C.ChatID = CC.ChatID 
  JOIN Users U 
    ON U.UID = C.FKFromUID 
 where C.FKToUID = 2 
union 
select U.UserID, FullName, Message from ChatMsgs C JOIN (select max(ChatID) chatID from ChatMsgs group 
    by FKToUID, FKFromUID) CC ON C.ChatID=CC.ChatID JOIN Users U ON U.UID=C.FKToUID where C.FKFromUID=2

Answer 1

如果我理解了您的问题，您只需选择所需的ChatMsgs（例如FKToUID = 2）并加入Users。

SELECT Users.UserID, Users.Fullname, ChatMsgs.Message
  FROM ChatMsgs
  INNER JOIN Users ON ChatMsgs.FKFromUID = Users.UserID
 WHERE ChatMsgs.FKToUID = 2

Answer 2

对于用户2，您可以获取相关聊天的最大ID和与用户的联接

  select User.UserID, Users.Fullname, t.Message
  from ( 
    select case when FKFromUID = 2 then FKToUID else FKFromUID 
            END as UserID 
    from ChatMsgs
    where id in (
   select max(id)
   from ChatMsgs where  FKFromUIDFromUID  = 2
   union 
   select max(id)
   from ChatMsgs where  FKToUID  = 2
   ) )  t 
   inner join users on t.UserID = Userse .UserID

Answer 3

我一直认为必须有一种更简单的方法......但这似乎有用。

样本： http://rextester.com/MUMAVP47179

SELECT U.userID, U.FullName, Cm.Message
FROM (SELECT MAX(Z.ChatID) chatID, UID
      FROM (SELECT ChatID, case when FKFROMUID = 2 then FKTOUID else FKFROMUID end as UID 
            FROM ChatMsgs) Z
      GROUP BY UID) Y
INNER JOIN Users U
 on U.userID = Y.UID
INNER JOIN ChatMsgs CM
 on Cm.ChatID = Y.ChatID 
ORDER BY Y.chatID desc;

Answer 4

select U.UserID, FullName, Message, C.CreationTime ChatTime from UserChats C JOIN (select max(UserChatID) chatID from UserChats group by FKToUserID, FKFromUserID) CC ON C.UserChatID=CC.ChatID JOIN Users U ON U.UserID=C.FKFromUserID where C.FKToUserID=@userID 
UNION 
select U.UserID, FullName, Message, C.CreationTime ChatTime from UserChats C JOIN (select max(UserChatID) chatID from UserChats group by FKFromUserID, FKToUserID) CC ON C.UserChatID=CC.ChatID JOIN Users U ON U.UserID=C.FKToUserID where C.FKFromUserID=@userID

MySQL查询以获取最新的唯一聊天对话

4 个答案: