Php MYSQL使用GROUP BY和ORDER BY返回奇怪的结果

尝试做一个论坛系统优化，在一个查询中选择所有最后的海报，并将它们存储在一个数组中。问题是数据库正在返回非预期的结果

PHP版本5.3.13

MySQL版本5.1.63

$getPosts = $dB->fetch('
  SELECT
    post_id, post_poster_id, post_topic_id, post_time,
    COUNT(post_id) as count
  FROM forum_posts
  WHERE post_topic_id IN (
    SELECT topic_id
    FROM forum_topics
    WHERE topic_forum_id = ' . $forum_id . '
  )
  GROUP BY post_topic_id
  ORDER BY post_time DESC
');
foreach($getPosts as $lastPoster)
{
    $lastPosts[$lastPoster['post_topic_id']] = $lastPoster;
}

---

您期望得到什么？请发布示例数据和所需输出。请详细说明。你所说的意外结果是什么意思？Group by将按

post\u topic\u id

@user1486625将行分组欢迎使用StackOverflow！请确保您的问题中包含了所有相关信息，

数据库返回了非预期的结果

并不能真正帮助我们回答问题，因为您没有告诉我们您得到了什么，为什么这是意外的，以及您期望的是什么。请在您的问题中添加更多信息，并请尽快查看，以便您了解我们对您的期望以及您对我们的期望。对于SQL问题，提供组也总是有帮助的。部分分组通常会产生“意外”结果。例如，

GROUP BY

子句中未指定的列不会按“预期”的方式排序。

SELECT
   p.post_id, p.post_poster_id, p.post_topic_id, p.post_time, l.count

FROM forum_posts AS p

-- joining with list of latests in a derived table
INNER JOIN

-- getting latest time and count
(SELECT p.post_topic_id, MAX(post_time) AS post_time, COUNT(post_id) AS count
FROM forum_posts AS p
-- filtering forum_id with join, join is usually preferrable than subquery
INNER JOIN forum_topics AS t ON t.topic_id = p.post_topic_id AND t.topic_forum_id = '$forum_id'
GROUP BY post_topic_id)

AS l USING(post_topic_id, post_time)

-- grouping to strip possible post_time-in-one-topic-duplicates
GROUP BY p.post_topic_id