Sql 如何根据查询区分记录与联合/分组记录

第一个高级别：

我有一个网页，它将显示用户和他或她接触过的每个人之间的对应关系，但只显示他们和其他人之间的最后一次对应关系，从而产生一个不同的用户列表

问题是我把所有的唱片都联合在一起，并将它们分组，但无法区分任何唱片。我不知道是谁发的消息，也不知道是电子邮件还是即时消息

我想创建一个虚拟列，如

'received' AS SentReceived
'sent' AS SentReceived
'email' AS CorrespondanceType
'instant_message' AS CorrespondanceType

但是我失去了理智

为了简洁起见，我将列出一些简化的sql，概述我的工作：

SELECT username, CorrespondanceType, SentReceived, MAX(Date) FROM (
SELECT username, 'email' AS CorrespondanceType, 'received' AS SentReceived, Date FROM tbl_email WHERE user_id='1'
SELECT username, 'email' AS CorrespondanceType, 'sent' AS SentReceived, Date FROM tbl_email WHERE user_id='1'
SELECT username, 'instant_message' AS CorrespondanceType, 'received' AS SentReceived, Date FROM tbl_instant_message WHERE user_id='1'
SELECT username, 'instant_message' AS CorrespondanceType, 'sent' AS SentReceived, Date FROM tbl_instant_message WHERE user_id='1'
) AS tbl_correspondance_summary
GROUP BY username

因此，如果我不使用伪列，我如何确定对应类型和发送它的人，如果我使用它们，我如何在用户名上显示结果

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这是SQL Server，顺便说一句

啊，很好，我做到了。刚刚做了编辑。谢谢！在您的表中，如何区分发送的消息和接收的消息？使用发送者id和接收者id，但实际上我将它们分别命名为用户id和成员id，用户id始终是登录用户，成员id是“另一个人”。我没有把这些放在上面只是为了简化这一切。在.netow中构建sql查询时，我通过在where子句中动态指定发送者或接收者来实现这一点，这是有效的。我还不完全明白，但它奏效了！谢谢

with RowNumbers as (
    SELECT username, CorrespondanceType, SentReceived, Date,
      ROW_NUMBER() OVER (partition by username order by Date desc) as RowNum
    FROM (
    SELECT username, 'email' AS CorrespondanceType, 'received' AS SentReceived, Date FROM tbl_email WHERE user_id='1'
    SELECT username, 'email' AS CorrespondanceType, 'sent' AS SentReceived, Date FROM tbl_email WHERE user_id='1'
    SELECT username, 'instant_message' AS CorrespondanceType, 'received' AS SentReceived, Date FROM tbl_instant_message WHERE user_id='1'
    SELECT username, 'instant_message' AS CorrespondanceType, 'sent' AS SentReceived, Date FROM tbl_instant_message WHERE user_id='1'
    ) AS tbl_correspondance_summary
)

select * from RowNumbers where RowNum = 1