依赖MySQL SELECT

Question

我的问题如下：

我的表格为MESSAGE和MESSAGE_COMMENT，

MESSAGE (id,content)

MESSAGE_COMMENT (id, message_id, content)

我需要为每封邮件选择所有邮件和 最多3条评论 ，例如：

type | id | content
M      15   "this is a message with no comments"
M      16   "this is another message with many comments"
R      16   "comment1"
R      16   "comment2"
R      16   "comment3"
M      17   "this is another message with no comments"

“id”在MESSAGE.id为消息时为COMMENT.message_id，在为评论时为{{1}}。

我希望我已经清楚地解释了我的问题..

Answer 1

SELECT  *
FROM    (
        SELECT  m.id,
                COALESCE(
                (
                SELECT  id
                FROM    message_comment mc
                WHERE   mc.message_id = m.id
                ORDER BY
                        mc.message_id DESC, id DESC
                LIMIT 2, 1
                ), 0) AS mid
        FROM    message m
        ) mo
LEFT JOIN
        message_comment mcd
ON      mcd.message_id >= mo.id
        AND mcd.message_id <= mo.id
        AND mcd.id >= mid

在message_comment (message_id, id)上创建一个索引，以便快速工作。

在我的博客中查看此文章，了解其工作原理的详细说明：

• Advanced row sampling

Answer 2

这就是因为PHP在您的服务器（侧面）中进行了解析，并且由它生成的HTML转到客户端浏览器并进行了渲染......

Answer 3

我不是工会的粉丝，但有时候他们有自己的位置...... :)。

SELECT type, id, content FROM ( 
SELECT 'M' AS type, id, 0 AS reply_id, content FROM MESSAGE
UNION
SELECT 'R' AS type, message_id AS id, id AS reply_id, content FROM MESSAGE_COMMENT) a 
ORDER BY id, reply_id

返回

+------+----+--------------------------------------------+
| type | id | content                                    |
+------+----+--------------------------------------------+
| M    | 15 | this is a message with no comments         |
| M    | 16 | this is another message with many comments |
| R    | 16 | comment1                                   |
| R    | 16 | comment2                                   |
| R    | 16 | comment3                                   |
| M    | 17 | this is another message with no comments   |
+------+----+--------------------------------------------+

注意：如果孤立的message_comments是一个问题，那么SELECT中的第二个UNION很容易通过INNER JOIN重新加工到MESSAGE表。