選擇SQL Server中每個唯一組合的最后一行

Question

我需要選擇每個唯一組合的最后一行，而不考慮組合元素的順序。 例如，組合(5,6)和(6,5)將被視為相同的組合。

這是表格上的數據：

我的預期輸出：

到目前為止，我所做的是：

Select * From
(
Select *, ROW_NUMBER() over (Partition by SenderId, ReceiverId order by MessageId desc) as Seq From Messages
) as TempTalbe Where Seq = 1

我的查詢的輸出是：

SQL Server專家有什么幫助嗎？

Answer 1

您可以為此使用row_number() 。 傳統方法是：

select m.*
from (select m.*,
             row_number() over (partition by (case when senderId < receiverId then senderId else receiverId end),
                                             (case when senderId < receiverId then receiverId else senderId end)
                                order by messageSentAt desc
                               ) as seqnum
      from messages m
     ) m
where seqnum = 1;

您也可以在不使用子查詢的情況下對此進行措辭：

select top (1) with ties m.*
from messages m
order by row_number() over (partition by (case when senderId < receiverId then senderId else receiverId end),
                                         (case when senderId < receiverId then receiverId else senderId end)
                            order by messageSentAt desc
                           );

Answer 2

嘗試這個：

select messageId,
       messageBody,
       messageSentAt,
       isRead
from (
    select messageId,
           messageBody,
           messageSentAt,
           isRead,
           row_number() over (partition by senderId, receiverId order by messageSentAt desc) rn
    from (
        select messageId,
               case when senderId > receiverId then senderId else receiverId end senderId,
               case when senderId < receiverId then senderId else receiverId end receiverId,
               messageBody,
               messageSentAt,
               isRead
        from Messages
    ) a
) a where rn = 1

最內部的查詢統一了發送者和接收者，因此它們的順序無關緊要：

case when senderId > receiverId then senderId else receiverId end senderId,
case when senderId < receiverId then senderId else receiverId end receiverId,

中間查詢為每行分配行號，因此第一個是最新日期。 大多數外部查詢只是根據該行號進行過濾。