select 
t2.cust_id,sum(t1.item_price*t1.quantity) as total_ordered
from OrderItems t1
left join Orders t2
on 
t1.order_num = t2.order_num
group by 
t2.cust_id
having  t2.cust_id is not NULL
order by 
total_ordered DESC

这里使用了Left join 的方式，来进行多表查询，选择OrderItems表作为驱动表，查询条件为 t1.order_num = t2.order_num

，找出符合条件的数据，但是还要保证右表的t2.cust_id不能为空。这是需要过滤的数据

，