select
cust_name
,order_num	
,(
    select
    sum(quantity*item_price) 
    from OrderItems o
     where o.order_num = ord.order_num
    group by order_num	
) as OrderTotal
from Customers c
join Orders ord
using(cust_id)
order by cust_name

涉及到要查询的字段，部分需要聚合，采用标量子查询，逻辑更清晰