I have a fairly 'standard' ecommerce data model

(See attached for full detail)

- id

- name

- product_id

- category_id

- id

- parent_id

- name

- path

- id

- order_id

- product_id

- amount

I'm trying to create a report: Sales by category.

What's the best way of doing this? Order items are joined to product, which in turn is joined to product category/