mysql - SQL从/到特定用户获取最后消息

Question

我正在尝试从表中进行 SQL 查询， - 从所有以用户 36 作为发件人或收件人的用户对获取最后一条消息，并将它们与用户表连接起来以获取名称。我已经设法创建了这样的东西，但仍然想问一下是否有更简单的 |有效的解决方案。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。更新。示例消息表中的数据:

mysql> SELECT id, sender_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 ids - 262, 263

Answer 1

我用的是SQL Server 2008，你不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来说似乎过于复杂。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

我不得不更改一些字段名称，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:现在您已经添加了更多信息，并显示消息表上有一个 id 字段，您可以使用类似这样的东西(注意:我有 SQL Server，所以您可能需要更改查询对于 MySQL):

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 找到最近的消息(基于 id，我假设它是一个自动递增数字)为每对有序的发件人/收件人用户 ID。其结果将重新加入 Fed_Messages 表，以确保我们获得正确的发件人/收件人姓名。

更新的 SQL fiddle :http://sqlfiddle.com/#!3/1f07a/2