What is the id of the order which has 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
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
[{'order_id': 15}]