Select multiple field duplicates from MySQL Database

Question

I\'ve got an old forum which contains threads with duplicate first posts (perhaps differing replies). I want to delete all but one of these threads (leaving the thread with the

Answer 1

A very hacky solution is to get the thread_id sorted by view_count in a GROUP_CONCAT. Then, we can utilize string operations to get the thread_id with minimum view_count.

In your SELECT clause, instead of t.thread_id, you can try the following:

SUBSTRING_INDEX(GROUP_CONCAT(DISTINCT t.thread_id 
                             ORDER BY t.view_count ASC 
                             SEPARATOR ','), 
                ',', 
                1) AS thread_id_with_minimum_views

Now, based on the SELECT query to identify Duplicate records with Minimum view, DELETE query to delete such records from the xf_thread table will be as follows:

DELETE t_delete FROM xf_thread AS t_delete 
INNER JOIN (SELECT CAST(SUBSTRING_INDEX(GROUP_CONCAT(DISTINCT t.thread_id ORDER BY t.view_count ASC SEPARATOR ','), ',', 1) AS UNSIGNED) AS tid_min_view 
            FROM (SELECT * FROM xf_thread) t 
            INNER JOIN xf_post p ON p.thread_id = t.thread_id 
            WHERE t.first_post_id = p.post_id 
              AND t.user_id = 0 
              AND t.reply_count < 2 
            GROUP BY t.title, t.username, p.message 
            HAVING Count(t.title) > 1 
               AND Count(t.username) > 1 
               AND Count(p.message) > 1 
            ORDER BY t.thread_id) AS t_dup 
  ON t_delete.thread_id = t_dup.tid_min_view