Who ordered the order with most items?
select t1.customer_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 its order id?
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