Changing the content of "Sub category" based on the selection made in "Category"

Hi,

I have a table where records have a category and a sub category. Both category and sub category are "Choice fields", but I want the choices presented in the Sub Category to depend on the selection made in the Category.

Example: 

Category A would have Sub Categories 1, 2 and 3
Category B would have Sub Categories 4 and 5
Category C would not have a Sub Category at all
Sub Category is empty by default, and only filled when Category is selected or changed.

I do not want to create a sub category field per option in the category, and hide the sub category fields that do not correspond with the selection made in Category. I want 1 Sub Category field, with choice options being presented based on the value of Category.

I don't really see how to easily do this, though, so if anyone has some experience with this, help would be very appreciated.

Cheers,

Jacques.