select c.cust_name,oi.order_num,
sum(oi.quantity*oi.item_price) as OrderTotal
from Orders as o
left join OrderItems as oi on o.order_num = oi.order_num
left join Customers as c on o.cust_id = c.cust_id

group by cust_name,order_num
order by cust_name,order_num