Question

表：

(`post_id`, `forum_id`, `topic_id`, `post_time`) 
(79, 8, 4, '2012-11-19 06:58:08');
(80, 3, 3, '2012-11-19 06:58:42'),
(81, 9, 9, '2012-11-19 06:59:04'),
(82, 11, 6, '2012-11-19 16:05:39'),
(83, 9, 9, '2012-11-19 16:07:46'),
(84, 9, 11, '2012-11-19 16:09:33'),

查询：

SELECT  post_id, forum_id, topic_id FROM posts 
GROUP BY topic_id 
ORDER BY post_time DESC
LIMIT 5

结果：

[0] => [post_id] => 84 [forum_id] => 9 [topic_id] => 11  
[1] => [post_id] => 82 [forum_id] => 11 [topic_id] => 6  
[2] => [post_id] => 81 [forum_id] => 9 [topic_id] => 9  
[3] => [post_id] => 80 [forum_id] => 3 [topic_id] => 3  
[4] => [post_id] => 79 [forum_id] => 8 [topic_id] => 4

问题：

如何重写查询以便返回post_id - ＆gt; 83而不是post_id - ＆gt; 81？

他们都有相同的论坛和主题ID，但是post_id - ＆gt; 81的日期早于post_id - ＆gt; 83.

但似乎Group By获得了“第一”记录，而不是“最新”记录。

我尝试将查询更改为

SELECT  post_id, forum_id, topic_id, MAX(post_time)

但是返回post_id 81和83

Answer 1

如果选择未在group子句中使用但未聚合的属性，则结果未指定。 I.e 您不知道从哪个行中选择其他属性。（sql标准不允许这样的查询，但MySQL更放松）。

然后应该写入查询，例如如

SELECT post_id, forum_id, topic_id
FROM posts p
WHERE post_time =
  (SELECT max(post_time) FROM posts p2
   WHERE p2.topic_id = p.topic_id
   AND p2.forum_id = p.forum_id)
GROUP BY forum_id, topic_id, post_id
ORDER BY post_time DESC
LIMIT 5;

或

SELECT post_id, forum_id, topic_id FROM posts
NATURAL JOIN
(SELECT forum_id, topic_id, max(post_time) AS post_time
 FROM posts
 GROUP BY forum_id, topic_id) p
ORDER BY post_time
LIMIT 5;

Answer 2

尝试类似

的内容

SELECT post_id, forum_id, topic_id 
FROM   (SELECT post_id, forum_id, topic_id
        FROM posts
        ORDER BY post_time DESC) 
GROUP BY topic_id 
ORDER BY topic_id desc
LIMIT 0,5

根据需要更改order by和limit。

Answer 3

也许不是最好的方法，但有时函数 group_concat（）可以是userfull，它将返回一个所有聚合值的字符串，按照你想要的方式排序并用逗号分隔（耦合值是由空间隔开）。然后我使用函数SPLIT_STRING（）来剪切字符串中的第一个id。

SELECT  
post_id, 
SPLIT_STRING( group_concat( forum_id, post_time ORDER BY post_time DESC ) ,' ',1 )as forum_id, 
SPLIT_STRING( group_concat( topic_id, post_time ORDER BY post_time DESC ) ,' ',1 )as topic_id ,
FROM posts 
GROUP BY topic_id 
ORDER BY post_time DESC
LIMIT 5

所以聚合的forum_id，post_time将是这样的：

81 2012-11-19 06：59：04,83 2012-11-19 16:07:46

所以你需要使用整数和日期时间对的字符串表示，每个情侣用逗号分隔，所以我用这个函数得到第一个INT：

CREATE FUNCTION SPLIT_STRING(str VARCHAR(255), delim VARCHAR(12), pos INT)
RETURNS VARCHAR(255)
RETURN REPLACE(SUBSTRING(SUBSTRING_INDEX(str, delim, pos),
       LENGTH(SUBSTRING_INDEX(str, delim, pos-1)) + 1),
       delim, '');

注意：函数SPLIT_STRING（str，delim，pos）在这里找到：Equivalent of explode() to work with strings in MySQL

Answer 4

这也适合你。

SELECT *
FROM (
  SELECT post_id, forum_id, topic_id FROM posts
  ORDER BY post_time DESC
  LIMIT 5
) customeTable
GROUP BY topic_id

php mysql Group By获取最新记录，而不是第一条记录

4 个答案: