题解 | #返回每个顾客不同订单的总金额#

-- 必须联表查询
select cust_id,total_ordered
from Orders o 
join (
    select order_num,sum(item_price *quantity) as total_ordered
    from OrderItems
    group by order_num
) t on o.order_num = t.order_num
order by total_ordered desc;

必须经过联表查询，才能筛选出结果。

使用聚合函数sum,必须进行分组group by。注意：聚合函数一般在select后面，这样想，就不会出错了。

子查询筛选的时候，记得给sum的结果命名，否则外层查询找不到total_ordered。