What are the names of the customers who bought product "food" at least once?
select t1.customer_name from customers as t1 join orders as t2 join order_items as t3 join products as t4 on t1.customer_id = t2.customer_id and t2.order_id = t3.order_id and t3.product_id = t4.product_id where t4.product_name = "food" group by t1.customer_id having count(*) >= 1