我试图使SQL查询表,-获取用户36作为发送者或接收者的所有用户对的最后消息,并将它们与users表连接以获取名称。我已经设法创建了这样的东西,但仍然想问是否有更简单高效的解决方案。Mysql版本- 5.5.31
table: messages
fields:
sender_user_id, recipient_user_id, date, text
查询 :
SELECT
*
FROM
(SELECT
(CASE sender_user_id > recipient_user_id
WHEN true THEN CONCAT(recipient_user_id, '|', sender_user_id)
WHEN false THEN CONCAT(sender_user_id, '|', recipient_user_id)
END) as hash,
sender_user_id,
recipient_user_id,
date,
text,
u.first_name
FROM
fed_messages
LEFT JOIN fed_users as u ON ((fed_messages.sender_user_id = 36 AND fed_messages.recipient_user_id = u.id) OR (fed_messages.recipient_user_id = 36 AND fed_messages.sender_user_id = u.id))
WHERE
sender_user_id = 36 OR recipient_user_id = 36
ORDER BY date DESC) as main
GROUP BY hash;
谢谢。乌利希期刊指南。来自示例消息表的数据:
mysql> SELECT id, send_user_id, recipient_user_id, text, date FROM federation。fed_messages WHERE id> 257;
-----+----------------+-------------------+-----------------+---------------------+
| id | sender_user_id | recipient_user_id | text | date |
+-----+----------------+-------------------+-----------------+---------------------+
| 258 | 40 | 36 | and one more | 2013-06-06 10:57:17 |
| 259 | 36 | 38 | another message | 2013-06-06 11:03:49 |
| 260 | 38 | 36 | some mes | 2013-06-06 12:29:33 |
| 261 | 38 | 36 | message | 2013-06-06 12:29:53 |
| 262 | 36 | 38 | message | 2013-06-06 12:47:26 |
| 263 | 36 | 40 | some message | 2013-06-10 16:22:46 |
结果应该是id - 262,263
我用的是SQL Server 2008,你不用说你用的是哪个数据库
从您提供的信息来看,您的查询对于您需要的输出似乎过于复杂。下面是一个简单的查询,用于获取涉及用户36的所有消息:
SELECT
sender.msg_user_name AS sender_user_name
,recipient.msg_user_name AS recipient_user_name
,msg_date
,msg_text
FROM
dbo.Fed_Messages
INNER JOIN dbo.Fed_User AS sender
ON sender.msg_user_id = sender_user_id
INNER JOIN dbo.Fed_User AS recipient
ON recipient.msg_user_id = recipient_user_id
WHERE
sender_user_id = 36
OR recipient_user_id = 36
ORDER BY
msg_date DESC
我不得不改变一些字段名,因为在SQL Server中,您选择的一些名称是保留字。
SQL Fiddle: http://sqlfiddle.com/#!3/b8e88/1
编辑:现在你已经添加了一些更多的信息,并显示在消息表上有一个id
字段,你可以使用这样的东西(注意:我有SQL Server,所以你可能不得不改变MySQL的查询):
SELECT sender.msg_user_name AS sender_user_name
,recipient.msg_user_name AS recipient_user_name
,msg_date
,msg_text
FROM dbo.Fed_Messages
INNER JOIN dbo.Fed_User AS sender ON sender.msg_user_id = sender_user_id
INNER JOIN dbo.Fed_User AS recipient ON recipient.msg_user_id = recipient_user_id
INNER JOIN ( SELECT MAX(id) AS most_recent_message_id
FROM dbo.Fed_Messages
GROUP BY CASE WHEN sender_user_id > recipient_user_id
THEN recipient_user_id
ELSE sender_user_id
END -- low_id
,CASE WHEN sender_user_id < recipient_user_id
THEN recipient_user_id
ELSE sender_user_id
END -- high_id
) T ON T.most_recent_message_id = dbo.Fed_Messages.id
WHERE sender_user_id = 36
OR recipient_user_id = 36
ORDER BY msg_date DESC
查询的FROM
部分中的SELECT
查找每个有序的发送者/接收者用户id对的最新消息(基于id
,我假设它是一个自动递增的数字)。结果将重新连接到Fed_Messages
表中,以确保我们获得正确的发送方/接收方名称。
更新SQL Fiddle: http://sqlfiddle.com/#!3/1f07a/2