这可以在一个快速的mysql查询中吗?

时间:2015-03-08 21:11:37

标签: mysql group-by

我有三张桌子,我需要不同的数据。可悲的是,我还需要能够提取最新的一行。

以下是我的表格:

消息:我只是将消息内容存储在一个表中,因为一个文本可以发送给多个用户

+------------+------------------+------+-----+---------+----------------+
| Field      | Type             | Null | Key | Default | Extra          |
+------------+------------------+------+-----+---------+----------------+
| message_id | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| content    | varchar(255)     | NO   |     | 0       |                |
+------------+------------------+------+-----+---------+----------------+

对话:此表仅反映两个用户之间的单个对话。

+-----------------+------------------+------+-----+---------+----------------+
| Field           | Type             | Null | Key | Default | Extra          |
+-----------------+------------------+------+-----+---------+----------------+
| partner_id      | int(10) unsigned | NO   | MUL | NULL    |                |
| conversation_id | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| expedition_id   | int(11)          | NO   |     | NULL    |                |
| active          | tinyint(4)       | NO   |     | 1       |                |
+-----------------+------------------+------+-----+---------+----------------+

conversation_messages :此表存储有关所交换实际消息的信息。

+-----------------+------------------+------+-----+---------+-------+
| Field           | Type             | Null | Key | Default | Extra |
+-----------------+------------------+------+-----+---------+-------+
| message_id      | int(11) unsigned | NO   | PRI | NULL    |       |
| receiver_id     | int(11) unsigned | NO   | PRI | NULL    |       |
| conversation_id | int(11) unsigned | NO   | MUL | NULL    |       |
| status          | tinyint(4)       | NO   |     | NULL    |       |
| timestamp       | datetime         | YES  |     | NULL    |       |
+-----------------+------------------+------+-----+---------+-------+

我现在想要做的是选择每个对话中的最新消息,并从此消息中获取内容。 (听起来很简单,但没有找到简单的解决方案)。我尝试的是以下内容:

SELECT max(c_m.message_id), m.content, c_m.`status` 
FROM expedition_conversations e_c, conversation_messages c_m 
    INNER JOIN messages m ON m.message_id = c_m.message_id 
WHERE e_c.expedition_id = 1 AND (c_m.conversation_id = e_c.conversation_id)      
GROUP BY c_m.conversation_id;

遗憾的是,由于GROUP BY内部似乎大部分时间都在选择第一个插入的行,因此从content表中选择的messages是错误的,而message_id已选中来自conversation_messages是正确的。

知道如何在一个查询中执行此操作吗?如果您有任何改变表结构的建议,我也很感激。

提前致谢。

3 个答案:

答案 0 :(得分:1)

这是可能的,因为您使用AUTO_INCREMENT表示最高ID属于最新消息:

SELECT
  messages.*,
FROM
  conversations
  INNER JOIN (
    SELECT conversation_id, MAX(message_id) AS maxmsgid
    FROM conversation_messages
    GROUP BY conversation_id
  ) AS latest ON latest.conversation_id=conversations.id
  INNER JOIN messages
    ON messages.message_id=latest.maxmsgid
WHERE 
  1=1 -- whatever you want or need!

由于此查询的速度非常慢,您可能需要考虑以下几个选项:

  • 向其投掷硬件:使用足够的RAM并配置MySQL以尽可能晚地转到临时表的磁盘
  • 使用非规范化并在ON AFTER INSERTmessages触发更新conversation_messages上包含最新消息ID的字段

答案 1 :(得分:1)

您可能想尝试此版本:

SELECT c_m.message_id, m.content, c_m.`status` 
FROM expedition_conversations e_c join
     conversation_messages c_m 
     ON c_m.conversation_id = e_c.conversation_id INNER JOIN
     messages m
     ON m.message_id = c_m.message_id 
WHERE e_c.expedition_id = 1 AND
      NOT EXISTS (SELECT 1
                  FROM conversation_messages cm2
                  WHERE cm2.conversation_id = c_m.conversation_id AND
                        cm2.timestamp > c_m.timestamp
                 ) 

为了提高性能,您需要conversation_messages(conversation_id, timestamp)上的索引。

答案 2 :(得分:0)

尝试这个小技巧:

SELECT c_m.message_id, m.content, c_m.status
FROM expedition_conversations e_c
JOIN (select * from (
    select * from conversation_messages
    order by message_id desc) x
  group by conversation_id) c_m ON c_m.conversation_id = e_c.conversation_id
INNER JOIN messages m ON m.message_id = c_m.message_id 
WHERE e_c.expedition_id = 1

这将适用于您的版本,如果mysql - 5.6.19 - 并且应该超出其他方法。