Nested Dynamic Lists

I'm trying to add the ability to attach a category and sub-category to a record. These lists of values need to be dynamic, so I am making use of the Choice (dynamic) field type. The category popup populates just fine, but I'm not sure what my query should be for sub-category. To clarify, category and sub-category are related tables; I associated sub-categories with their parent category creating a hierarchy. Any suggestions on what my sub-category query should look like? From what I understand, it basically needs to query the sub-category table where its category (reference) equals the current record's category name. Thanks for any help with this.