История изменений

select * from forum f 
left join (select substring_index(thread_path, '.', 1) as root,
             max(comment_id) as max 
             from forum group by root
          ) o on substring_index(f.thread_path, '.', 1)=o.root 
order by o.max desc, f.thread_path

select * from forum f 
left join (select substring_index(thread_path, '.', 1) as root,
             max(comment_id) as max from forum group by root
          ) o on substring_index(f.thread_path, '.', 1)=o.root 
order by o.max desc, f.thread_path