Star schema join and rollup

You have a star schema: a fact table sales as rows [product_id, amount] (amount int), and a dimension table products as rows [product_id, category] (category string). Join the fact rows to their product's category and roll up total amount per category. Fact rows whose product_id is missing from the dimension table are dropped (inner join). Return [category, total_amount] pairs sorted by category ascending. product_ids in the dimension table are unique.