Which customer bought the most items? | Do you want the name of the customer? | That would be lovely.
SELECT t1.customer_name FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id   =   t2.customer_id JOIN order_items AS t3 ON t2.order_id   =   t3.order_id GROUP BY t1.customer_name ORDER BY sum ( t3.order_quantity )  DESC LIMIT 1
most items refers to MAX(SUM(order_quantity)); customer who bought refers to linking customers, customer_orders, and order_items tables; name of the customer refers to customer_name;