Needing help with sub-boards

[alphawolf21]

So I've gotten sub-boards established on my PHP/SQL forum, and they work great, with the exception of one thing. To check for last posts, I search my topics table for the most recent ID entry for that particular board. The problem with this is when these boards have sub-boards. I've added in a feature so members can click on "Expand" and it will show all sub-boards (along with their last posts), but while hidden, the only last post that shows is just of the parent board. How would I find which is the last post through all sub-boards of the parent board, and compare it/them to the parent board?

 

Thanks