SQL从/向特定用户获取最后一条消息

Question

我正在尝试从表中进行SQL查询，-从所有对用户中以用户36作为发送者或接收者的最后一条消息，并将它们与用户表结合起来以获取名称。我已经设法创建了这样的东西，但仍然想问一下是否还有更简单的html" target="_blank">方法。有效的解决方案。MySQL版本-5.5.31

    table: messages
    fields: 
    sender_user_id, recipient_user_id, date, text

询问：

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。样本消息表中的数据：

mysql> SELECT id，sender_user_id，receiver_user_id，文本，日期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 |

结果应为id-262、263

Answer 1

我正在使用SQL Server 2008，您没有说要使用哪个数据库。

根据您提供的信息，查询对于所需的输出而言似乎过于复杂。这是一个简单的查询，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

我不得不更改某些字段名称，因为在SQL Server中，您选择的某些名称是保留字。

SQL小提琴：http：//sqlfiddle.com/#！3 / b8e88 /
1

编辑： 现在您已经添加了更多信息，并且显示id了消息表上有一个字段，您可以使用类似以下的内容（注意：我有SQL
Server，因此您可能必须更改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

将SELECT在FROM查询的一部分查找最近的消息（根据id，我假设它是一个自动递增的数字），每个有序对发件人/接受者用户ID的。结果将重新连接到Fed_Messages表中，以确保我们正确获取发送者/接收者的名称。

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