Question

我有这些表格。

用户：

|ID|Field1....|

UserMessages ：

|ID|UserSenderId|UserReceiverId|Message|CreatedOn|

其中UserSenderId和UserReceiverId是PK User.Id

的FK

我想选择用户之间每次对话的第一条消息（比如左边的Facebook，我看到一个用户列表和最后一条消息的第一个字符）

我尝试了这个查询，但我为同一对用户提供了多个结果

SELECT DISTINCT 
    UserMessages.* 
FROM 
    UserMessages 
JOIN 
    (SELECT 
         MAX(UserMessages.Id) AS IDMAX,
         UserMessages.IdReceiver,
         UserMessages.IdSender 
     FROM 
         UserMessages 
     WHERE 
         UserMessages.IdReceiver = @UserId OR UserMessages.IdSender = @UserId 
     GROUP BY 
         UserMessages.IdReceiver, UserMessages.IdSender) IDMAX ON IDMax.IDMAX = UserMessages.Id

示例

UserMessages：

|ID|UserSenderId|UserReceiverId|Message|
  1|46|47|Hello
  2|47|46|Hello!
  3|46|48|Hey!!
  4|50|46|How are you?
  5|51|49| Hello

考虑到记录的userid = 46，我想只选择标识为2 3 4的消息，其中id = 2是用户46和47之间的最后一个消息（记录的用户可以是发送者或接收者）

谢谢！

Answer 1

您可以使用row_number()执行此操作。诀窍是无论发送者和接收者是谁，都要配对消息。您可以先将较小的ID放入，然后再放入较大的ID：

select um.*
from (SELECT um.*,
             row_number() over (partition by (case when idReceiver < idSender then idReceiver else idSender end),
                                             (case when idReceiver < idSender then idSender else idReceiver end)
                                order by id desc
                               ) as seqnum
     from UserMessages um
     where um.IdReceiver = @UserId OR um.IdSender = @UserId
    ) um
where seqnum = 1;

T-SQL Group具有独特的价值

1 个答案: