如何解决如何获取唯一值以比较同一表的两列?
当前,我正在使用SQLite进行Django项目,在其中创建消息聊天框,并希望获取已登录用户的最新消息(无论发送还是接收)。
id receiver_id sender_id message_content created_at
1 1 2 some text 2020-08-11 13:29:47.342944
3 3 2 some text 2020-08-11 13:44:55.499638
4 2 1 some text 2020-08-11 14:20:55.499638
5 1 2 some text 2020-08-12 05:06:05.497500
6 2 5 some text 2020-08-12 10:39:31.234082
7 4 1 some text 2020-08-14 13:25:19.357876
使用下面的SQL查询之后。
SELECT max(created_at),*
FROM hireo_messages
WHERE receiver_id=2 or sender_id=2
GROUP BY receiver_id,sender_id
ORDER BY created_at DESC
我得到了以下结果。
id receiver_id sender_id message_content created_at
6 2 5 some text 2020-08-12 10:39:31.234082
5 1 2 some text 2020-08-12 05:06:05.497500
4 2 1 some text 2020-08-11 14:20:55.499638
3 3 2 some text 2020-08-11 13:44:55.499638
您会看到ID 5、4彼此聊天。因此,我想获取除ID 4以外的所有记录,因为2个用户之间的最新聊天是在ID 5中。这与Facebook Messenger仪表板中使用的概念相同。 请指导我是通过查询解决还是以其他方式解决。预先感谢您的帮助!
解决方法
我想你想要
SELECT LEAST(receiver_id,sender_id),GREATEST(receiver_id,MAX(created_at)
FROM hireo_messages
WHERE 2 IN (receiver_id,sender_id)
GROUP BY LEAST(receiver_id,sender_id)
ORDER BY MAX(created_at) DESC;
如果需要完整的行,请使用窗口功能:
SELECT m.*
FROM (SELECT m.*,ROW_NUMBER() OVER (PARTITION BY LEAST(receiver_id,sender_id) ORDER BY created_at DESC) as seqnum
FROM hireo_messages m
WHERE 2 IN (receiver_id,sender_id)
) m
WHERE seqnum = 1
ORDER BY MAX(created_at) DESC;
,
您可以使用窗口功能MAX()
:
select id,receiver_id,sender_id,message_content,created_at
from (
select *,max(created_at) over (partition by min(receiver_id,max(receiver_id,sender_id)) last_date
from hireo_messages
where 2 in (receiver_id,sender_id)
)
where created_at = last_date
order by id desc
如果您的SQLite版本不支持窗口功能,则可以使用NOT EXISTS
:
select h.* from hireo_messages h
where 2 in (receiver_id,sender_id)
and not exists (
select 1 from hireo_messages
where min(receiver_id,sender_id) = min(h.receiver_id,h.sender_id)
and max(receiver_id,sender_id) = max(h.receiver_id,h.sender_id)
and created_at > h.created_at
)
order by h.id desc
请参见demo。
结果:
| id | receiver_id | sender_id | message_content | created_at |
| --- | ----------- | --------- | --------------- | -------------------------- |
| 6 | 2 | 5 | some text | 2020-08-12 10:39:31.234082 |
| 5 | 1 | 2 | some text | 2020-08-12 05:06:05.497500 |
| 3 | 3 | 2 | some text | 2020-08-11 13:44:55.499638 |
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