select c.cust_name cust_name,SUM(m.item_price*m.quantity) total_price 
from Orders o 
inner join Customers c on o.cust_id=c.cust_id  
inner join OrderItems m on m.order_num=o.order_num 
group by c.cust_name having SUM(m.item_price*m.quantity) >='1000'
order by total_price