How do I place threads at the top(bump) based on last user post?

http://www.vidyasocks.com/requests.php

I want it so when I post in a thread it will bump that thread to the top.

This is what I currently have for the page"SELECT * from forum ORDER BY id DESC LIMIT 12";

The database has thread with an id(pk) and replies with a id(pk) and a thread_id(fk)

What do I do? Some kind of inner join I'd assume?

Solution

Do you have a date/timestamp for when threads and replies are inserted? You can't just order based on reply ID because there are threads without any reply and it isn't possible to correlate thread_id order with the reply id order.

If I have the right idea for your table structure you'd be doing something like this given some timestamps:

 SELECT DISTINCT forum.* FROM forum 
      LEFT JOIN replies on
          forum.id = replies.thread_id
  ORDER BY coalesce(replies.reply_timestamp, forum.thread_timestamp) DESC
    LIMIT 12

The coalesce would return replies.reply_timestamp if a reply exists otherwise the forum.thread_timestamp would be used.

edit: added DISTINCT based on Ami's comment