Well, it is still not clear what the OP means with 'categories' and apparently his problem is not so urgent that he cares to explain it to us. I assume that he means 'tables' though.
Now I am not really a MySQL crack, but I thought foreign key were needed to ensure referential integrety. That is, (among other things) you cannot delete a record from one table which is used (referred to) in another table.
It doesn't help for entries which are non-unique across tables. What is the problem with that anyway?