What is the id of the order which has the most items?
create table orders (order_id varchar, PRIMARY KEY (order_id)); create table order_items (order_id varchar, PRIMARY KEY (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