1

我正在尝试创建 SQL 来检索论坛线程的最新帖子列表。我有以下代码：

SELECT
 item_discuss_thread_id
 , item_discuss_post_title
 , COUNT(item_discuss_thread_id) AS nb_posts
FROM
 item_discuss_posts
GROUP BY
 item_discuss_thread_id

显然，这将在不考虑帖子是否最新的情况下进行分组。item_discuss_post_title只会获得组中的第一行。

我想知道有没有办法解决这个问题？如果没有，解决问题的最佳方法是什么……只有子查询？

谢谢，帕维尔

更新：请注意我需要所有线程，LIMIT 1 不能解决问题。ORDER BY 也不是一个选项，因为 GROUP BY 无论如何都会从组中选择第一条记录。这不是一个看起来那么简单的问题。

更新：

我真的想尽量避免使用子查询，或者如果这样做 - 尽可能使用它。我目前带来的是这样的：

SELECT
  ordered_by_date.item_discuss_thread_id
  , item_discuss_post_title
  , COUNT(item_discuss_thread_id) AS nb_posts
FROM
  (
   SELECT
     item_discuss_thread_id
     , item_discuss_post_title
   FROM
      item_discuss_posts
   ORDER BY
      item_discuss_post_datetime DESC
  ) AS ordered_by_date
GROUP BY
   item_discuss_thread_id

EXPLAIN EXTENDED 给出以下结果：

id, select_type, table, type, possible_keys, key, key_len, ref, rows, Extra
1, PRIMARY, <derived2>, ALL, \N, \N, \N, \N, 20, Using temporary; Using filesort
2, DERIVED, item_discuss_posts, index, \N, item_discuss_post_datetime, 8, \N, 20,
4

2 回答 2

2

好的，我自己提出了解决方案。我使用了一个依赖子查询来解决。这就是我的结果：

        SELECT
             item_discuss_threads.item_discuss_thread_id
             , item_discuss_threads.item_discuss_thread_datetime
             , item_discuss_threads.item_discuss_thread_title
             , latest_posts.item_discuss_post_title
             , latest_posts.item_discuss_post_datetime
             , COUNT(item_discuss_posts.item_discuss_post_id) AS nb_posts
        FROM
             item_discuss_threads
        INNER JOIN item_discuss_posts
             ON item_discuss_threads.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
        INNER JOIN item_discuss_posts AS latest_posts
             ON latest_posts.item_discuss_thread_id=item_discuss_threads.item_discuss_thread_id
        WHERE
             (
                  SELECT
                        item_discuss_post_id
                  FROM
                        item_discuss_posts AS p
                  WHERE
                        p.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
                  ORDER BY
                        item_discuss_post_datetime DESC
                  LIMIT
                       1
             )=latest_posts.item_discuss_post_id
        GROUP BY
             item_discuss_threads.item_discuss_thread_id
        ORDER BY
            latest_posts.item_discuss_post_datetime DESC
于 2009-12-09T23:22:17.090 回答
1

试试这个。

SELECT
  *
FROM
  (SELECT item_discuss_thread_id, item_discuss_post_title, COUNT(item_discuss_thread_id) AS nb_posts
   FROM item_discuss_posts
   ORDER BY __datecolumn__)
  AS ordered_by_date
GROUP BY
  ordered_by_date.item_discuss_thread_id

替换__datecolumn__为存储发布时间的列。

于 2009-12-08T20:44:00.183 回答