รหัสลูกค้าของลูกค้าที่มีคำสั่งซื้อมากที่สุดคืออะไร?ไม่จำเป็นต้องใช้ตารางทั้งหมด
What is the customer id of the customer who has the most orders?It is not neccessary to use all the tables.
 orders:  order_id [ number]  customer_id [ number]  order_status [ text]  date_order_placed [ time]  order_details [ text] 
SELECT T1.customer_id 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