Question

I am working on a forum system and I need to delete all topics and posts from a particular forum.

There is a posts table that holds all the posts. Each post has topic_id field which is the ID of the topic to which it belongs. The topics reside in topics table and each topic has forum_id field which is the ID of the forum it belongs to.

So now when I decide to delete forum with ID 1, how would I go about deleting all topics and posts from this forum as well?

Answer 1

First of all I recommend you take a good look at your data structure: If you use foreign key constraints of the ON DELETE CASCADE flavour, your sub-deletes will automatically be done for you – and this just one of the advantages you get.

If for some reasons you can’t or don’t want to employ such constraints, start by taking a look at how you would select the relvant topics and posts. This should give you quite a good start.

In your particular example something like

DELETE FROM posts
WHERE topic_id IN (
     SELECT id FROM topics WHERE forum_id=[id-to-delete]
  )

and then

DELETE FROM topics WHERE forum_id=[id-to-delete]

should do the trick.

Please understand, that I don’t recommend that. Use data consistency tools for data consistency.