题解 | #返回每个顾客不同订单的总金额#

分析：
根据题意，将第一个表进行分组查询，并计算总金额，将查询结果做为一张 表 与 Orders 表进行连接查询

• Orders表别名为 t1
• select order_num, sum(item_price * quantity) as total_ordered from OrderItems group by order_num 的结果，做为表 t2

select 
    cust_id,
    total_ordered
from
    Orders t1
inner join (select order_num, sum(item_price * quantity) as total_ordered 
from OrderItems group by order_num) t2
on 
    t1.order_num = t2.order_num
order by 
    total_ordered desc