题解 | 返回每个顾客不同订单的总金额

# select order_num, sum(item * quantity) total_ordered
# from OrderItems
# group by order_num
# order by total_ordered;

select Orders.cust_id, temp.total_ordered
from (
    select order_num, sum(item_price * quantity) total_ordered
    from OrderItems
    group by order_num
)  temp,  Orders
where temp.order_num = Orders.order_num
order by temp.total_ordered desc;

我的思路：先分组求出订单的总数，然后把这个结果当成一张临时表temp，表里就两个字段：order_num和total_ordered，然后问题就转换为从Orders表和temp表连表查询了，当然继续子查询也可以