Return multiple rows from matching on multiple CASE matches?
A join would be the best solution here.

Create a table that contains a list of categories and which products are associated with them, something like Product, Category we'll call it ProductCategories. Then join on this table.

SELECT p.Product, p.Quantity_Sold, pc.Category
FROM sales_table p
JOIN ProductCategories pc ON pc.Product = p.Product'


This has the added benefit of not having to modify your case statement as you add more categories (which quickly gets difficult/impossible to maintain), and only requires a simple insert into the new table.