SELECT cus.cust_name, o1.order_num, OrderTotal
from  Orders o1
LEFT JOIN Customers cus on cus.cust_id = o1.cust_id
LEFT JOIN (
SELECT order_num, sum(quantity*item_price) as OrderTotal
FROM OrderItems
GROUP BY order_num) as t1 on o1.order_num = t1.order_num
ORDER BY cust_name, order_num asc