题解 | #返回每个顾客不同订单的总金额#

#子查询
select cust_id,total_order
from Orders,(select order_num,sum(item_price*quantity) as total_order from OrderItems
group by order_num)as tt
where tt.order_num=Orders.order_num
order by total_order desc
#自然连接
select cust_id,total_order
from Orders 
natural join
(select order_num,sum(item_price*quantity) as total_order from OrderItems
group by order_num)as tt
where tt.order_num=Orders.order_num
order by total_order desc
#左连接
select cust_id,total_order
from Orders 
left join
(select order_num,sum(item_price*quantity) as total_order from OrderItems
group by order_num)as tt
on tt.order_num=Orders.order_num
order by total_order desc
#内连接
select cust_id,total_order
from Orders 
inner join
(select order_num,sum(item_price*quantity) as total_order from OrderItems
group by order_num)as tt
on tt.order_num=Orders.order_num
order by total_order desc