What is the product ID for food?
select product_id from products where product_name = 'food'
What are the order IDs that have those product IDs?
select t2.order_id from products as t1 join order_items as t2 where t1.product_id = t2.product_id and t1.product_name = 'food'
What customer IDs have those order IDs?
select t3.customer_id from products as t1 join order_items as t2 join orders as t3 where t1.product_id = t2.product_id and t1.product_name = 'food' and t3.order_id = t2.order_id