Adding Sub-categories To Database Application?

I am working on a inventory application and I have a question, I have created a table for each of the main categories that I will be using, now if I wanted to add sub categories would I create another table for the sub directory and set the type as a child for its corresponding category or would it be easier to keep the main categories and just add another column in each table for "sub category" so it uses the same table but I can just filter it out easier?

If it is a better option for creating parent and child tables, how would I go about listing them in a hierarchical treeview to show parent and child sub-categories in them.