SELECT b.customer_id, b.cust_last_name || ', ' || b.cust_first_name cust_name, SUM(a.ORDER_TOTAL) order_total
FROM demo_orders a, DEMO_CUSTOMERS b
WHERE a.customer_id = b.customer_id
GROUP BY b.customer_id, b.cust_last_name || ', ' || b.cust_first_name
ORDER BY NVL(SUM(a.ORDER_TOTAL),0) DESC