题解 | #返回每个顾客不同订单的总金额#

/*使用相关子查询*/
# 查询某个顾客的所有订单的订单总金额
select sum(item_price*quantity) 
from OrderItems oi
join Orders o
on oi.order_num=o.order_num
where cust_id=cust_id_num  # 其中cust_id_num从主查询中获得
# 查询各顾客已订购的总金额
select cust_id, (
    select sum(item_price*quantity) 
    from OrderItems oi
    join Orders o
    on oi.order_num=o.order_num
    where cust_id=Orders.cust_id 
) total_ordered
from Orders
order by total_ordered desc;