What is the id of the order with the most items?
select t1.order_id from orders as t1 join order_items as t2 on t1.order_id = t2.order_id group by t1.order_id order by count(*) desc limit 1
What is the id of the customer who ordered it?
select t1.customer_name from customers as t1 join orders as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count(*) desc limit 1