MySQL - how to show the latest topic per thread

问题

I am trying to create SQL for retrieveing a list of latests posts for the forum thread. I have the following code:

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

Obviously this will group without the respect of if the post is latest or not. item_discuss_post_title will just get the first row in the group.

I wonder if there's some way around this? If not, what is the best way to solve the problem... only subqueries?

Thanks, Pavel

UPDATE: Please note that I need all threads, LIMIT 1 is not solving the problem. Also ORDER BY is not an option as GROUP BY will select the first record from group anyway. This is not such a simple question as it can seem to be.

UPDATE:

I really want to try to avoid using subqueries or if doing so - use it the optimal may. What I came with currently is something like this:

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 gives the following result:

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,

回答1:

Try this.

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

Replace __datecolumn__ with the column that stores posting time.

回答2:

Ok, I came with solution myself. I used a dependent subquery to solve. This is what I have in the result:

        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

来源：https://stackoverflow.com/questions/1869160/mysql-how-to-show-the-latest-topic-per-thread

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