将多行合并为一行，在 SQL 服务器中的两列之间交换相同的值

Question

我正在开发一个基于 API 的简单聊天模块。我正在尝试获取特定用户的聊天对话，但由于 2 列彼此之间交换了相同的值，导致我的数据重复。

我想合并具有在 2 列之间交换的相同值的行，并且合并的行应基于数据库中插入的最新条目。

数据如下所示：

Id  To  From   Message        ConversationTime
1   1    2     hello              11:00AM
2   3    1     hi                 12:00PM
3   1    3     how are you?       12:15PM
4   3    1     I am fine.         12:30PM
5   4    5     Hi!                04:30PM
6   5    4     Hello              04:35PM
7   1    5     Hola!              06:30PM

例如，如果用户 ID 为 1，我的结果需要如下所示：

Id  To  From   Message        ConversationTime
1   1    2     hello              11:00AM
4   3    1     I am fine.         12:30PM
7   1    5     Hola!              06:30PM

如果 Id 是 5 那么结果将是这样的：

Id  To  From   Message        ConversationTime
6   5    4     Hello              04:35PM
7   1    5     Hola!              06:30PM

我的结果集如下所示：

Id  To  From   Message        ConversationTime
1   1    2     hello              11:00AM
3   1    3     how are you?       12:15PM
4   3    1     I am fine.         12:30PM
7   1    5     Hola!              06:30PM

任何帮助将不胜感激。提前致谢！

Answer 1

想法与链接副本Get top 1 row of each group 相同；只需使用 CASE 表达式即可获取 other 用户的 ID：

DECLARE @ID int = 1;
WITH RNs AS(
    SELECT ID,
           [To], --TO is a reserved keyword and should not be used for object names
           [From], --FROM is a reserved keyword and should not be used for object names
           Message,
           ConversationTime, --I assume this is a time
           ROW_NUMBER() OVER (PARTITION BY CASE [To] WHEN @ID THEN [From] ELSE [To] END ORDER BY ConversationTime DESC) AS RN --TO and FROM are reserved keywords and should not be used for object names
    FROM dbo.YourTable
    WHERE @ID IN ([To],[From])) --TO and FROM are reserved keywords and should not be used for object names
SELECT ID,
       [To], --TO is a reserved keyword and should not be used for object names
       [From], --FROM is a reserved keyword and should not be used for object names
       Message,
       ConversationTime --I assume this is a time
FROM RN
WHERE RN = 1;

Answer 2

SQL 服务器允许您在没有 case 表达式的情况下通过反透视数据然后使用 window 函数来执行此操作：

select t.*
from (select t.*,
             row_number() over (partition by v.user_other order by t.conversationTime desc) as seqnum
      from t cross apply
           (values (t.to, t.from), (t.from, to.to)
           ) v(user, user_other)
      where v.user = 1
     ) t
where seqnum = 1;

将多行合并为一行，在 SQL 服务器中的两列之间交换相同的值

Merge Multiple Rows to One Row having Same value swapped between 2 columns In SQL Server

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