Question

所以，我有一个表，主题

ThreadNumber | Sticky | Title  
1            | 1      | Read This Before Posting!  
2            | 0      | Second Topic  
3            | 0      | Check This

还有第二张表帖子

ThreadNumber | Author | Timestamp | Body                        | PostNumber  
1            | User1  | 1         | Read up!                    | 0  
1            | User2  | 2         | I see.                      | 1  
2            | User2  | 3         | So tell me what'chu want!   | 0  
3            | User3  | 5         | Yeah, check this out.       | 0  
2            | User3  | 7         | What'chu really really want!| 1  
2            | User1  | 10        | I'll tell you what I want!  | 2

（Posts中的时间戳是当给定用户向db提交注释时PHP time()函数给出的时间的十位二次整数表示。）所以，当涉及到显示所有论坛帖子时论坛概述页面上的主题，我想要做的是按照发布最后一条评论的顺序显示所有主题。我已经尝试了自己的嵌套MySQL查询（没有解决），所以我开始尝试做一些研究。我找到的任何其他东西都无法让我得到我想要的东西。我觉得this接近了，但我仍然得到了独特的ThreadNumbers给出的所有结果。理想情况下，我会得到的回报是：

ThreadNumber | Title                     | LastTimestamp  
2            | Second Topic              | 10  
3            | Check This                | 5  
1            | Read This Before Posting! | 1

非常感谢您阅读本书以及任何可能的[并且非常需要]帮助！

我试过了

    SELECT PostNumber, ThreadNumber, Body, Author, Timestamp
FROM (SELECT PostNumber, ThreadNumber, Body, Author, Timestamp
      FROM Posts
      ORDER BY Timestamp DESC) AS p
GROUP BY ThreadNumber;

它不仅不会返回我真正想要的东西，而且还没有像我预期的那样工作。

Answer 1

您可以通过以下示例在一个请求中执行此操作

select 
  Posts.ThreadNumber, 
  Title, 
  max(Timestamp) 
from Threads 
left join Posts on Threads.ThreadNumber = Posts.ThreadNumber  
group by Posts.ThreadNumber, Title 
order by max(timestamp) desc;

使用

等源数据

mysql> select * from Posts;
+--------------+--------+-----------+------+------------+
| ThreadNumber | Author | Timestamp | Body | PostNumber |
+--------------+--------+-----------+------+------------+
|            1 | User1  |        20 |      |          0 |
|            2 | User1  |        30 |      |          0 |
|            3 | User1  |        10 |      |          0 |
|            1 | User1  |        24 |      |          0 |
|            2 | User1  |         2 |      |          0 |
|            3 | User1  |       200 |      |          0 |
+--------------+--------+-----------+------+------------+
6 rows in set (0,00 sec)

和

mysql> select * from Threads;
+--------------+--------+----------+
| ThreadNumber | Sticky | Title    |
+--------------+--------+----------+
|            1 |      0 | Thread 1 |
|            2 |      0 | Thread 2 |
|            3 |      0 | Thread 3 |
+--------------+--------+----------+
3 rows in set (0,00 sec)

将输出以下

mysql> select distinct Posts.ThreadNumber, Title, max(Timestamp) from Threads left join Posts on Threads.ThreadNumber = Posts.ThreadNumber  group by Posts.ThreadNumber, Title order by max(timestamp) desc;
+--------------+----------+----------------+
| ThreadNumber | Title    | max(Timestamp) |
+--------------+----------+----------------+
|            3 | Thread 3 |            200 |
|            2 | Thread 2 |             30 |
|            1 | Thread 1 |             24 |
+--------------+----------+----------------+
3 rows in set (0,00 sec)

但在你的情况下，我将重新开发数据库结构。你的结构中有一些非常糟糕的问题。至少你可以在帖子中添加id字段。现在，您无法在其中设置有效索引，并且对于大数据，您的请求将会变慢。

按最新帖子的顺序拉出论坛帖子

1 个答案: