Relational database design would have you create a new table called ProductCategories that would have:
ProductCategoryID - Autonumber
ProductID - ProductID of product
CategoryID - CategoryID of product
That way a single product could have any number of product categories.
Then you would delete your CategoryID column from your products table and use this new table to maintain the relationship of products to categories.
The webassist Server Behavior "Manage Relational Table" would then be used to maintain the ProductCategories table from the product update page.
Then you use INNER JOINS to join the tables and display results for a given category. These concepts can be a bit tricky the first few times you do it, so I'd suggest a premier support ticket the first time to save you a lot of effort getting up to speed.