Question: What was the product id for the customer that ordered the most product quantity?
SQL: SELECT  T1.customer_id FROM customers AS T1 JOIN customer_orders AS T2 ON T1.customer_id   =   T2.customer_id JOIN order_items AS T3 ON T3.order_id   =   T2.order_id GROUP BY T1.customer_id ORDER BY sum ( T3.order_quantity )  DESC LIMIT 1