Question

我想根据两列选择distinct，这样如果两列中的任何一列具有相同的值，那么它只会发布一次该行。如果它有助于你理解，我正在做一个发送者/接收者的事情，我只想拉一行（无论是发送者还是接收者）。

以下是我希望MySQL工作的方式：

SELECT DISTINCT(sender AND receiver), message, timestamp  
FROM messages 
WHERE receiver=receiver_name OR sender=sender_name;

示例：

消息表：

id|  sender |receiver | message                | timestamp
1 | Jeffrey | Michael | Hey man, what's up     | 12:00 PM  
2 | Michael | Jeffrey | Not much. How are you? | 12:02 PM

SQL查询后的结果：

迈克尔的最新消息：不多。你好吗？时间：下午12:02

我希望这有点道理

Answer 1

从OP评论到Gordon Linoff的答案，似乎他有兴趣为他和其他人之间的每次转换检索最后一条消息，在这种情况下，发送者或接收者将是他

SET @myname = 'Michael' --just stick here your name

SELECT m.*
FROM   messages m
       INNER JOIN (SELECT MAX(ID) ID
                   FROM   messages
                   WHERE  @myname IN (receiver, sender)
                   GROUP BY COALESCE(NULLIF(receiver, @myname), sender)
                  ) a ON m.ID = a.ID

公式COALESCE(NULLIF(receiver, @myname), sender)将始终以另一个名称解析：

如果OP是发件人，那么NULLIF(receiver, @myname)将返回接收者，COALESCE
如果OP是接收方，那么NULLIF(receiver, @myname)将返回NULL而COALESCE将返回发件人

Answer 2

您需要使用concat函数。

SELECT distinct concat(sender,receiver)
FROM messages
WHERE receiver=receiver_name OR sender=sender_name;

有关详细信息，请参阅here。

Answer 3

您的代码 -

SELECT DISTINCT(sender AND receiver), message, timestamp 
FROM messages 
WHERE receiver=receiver_name OR sender=sender_name;

新代码 -

SELECT DISTINCT (sender , receiver), message, timestamp  
FROM messages 
WHERE .........

Answer 4

我想您要使用least()和greatest()：

select distinct least(sender, receiver), greatest(sender, receiver)
from messages;

如果您想要最新的时间戳，请使用group by：

select least(sender, receiver), greatest(sender, receiver),
       max(timestamp)
from messages
group by least(sender, receiver), greatest(sender, receiver);

编辑：

如果您想了解某人发送或接收的最后一封邮件的信息，您可以执行以下操作：

select m.*
from messages m
where not exists (select 1
                  from messages m2
                  where (m2.sender = m.sender or m2.receiver = m.receiver or
                         m2.receiver = m.receiver or m2.receiver = m.sender
                        ) and
                        m2.timestamp > m.timestamp
                 );

如何在SELECT DISTINCT中使用AND语句？

4 个答案: