select Customers.cust_name ,sum(OrderItems.item_price*OrderItems.quantity)as sumcount
from Customers  join Orders
on Customers.cust_id = Orders.cust_id
join OrderItems on Orders.order_num = OrderItems.order_num
group by Customers.cust_name
having sumcount>=1000

sum goup by having

不能用where是因为它不能接聚合函数