select order_id,customer_name,order_date
from(
select o.order_id,c.customer_name,o.order_date,row_number() over(partition by c.customer_id order by o.order_date desc) as rk
from orders o
join customers c
on o.customer_id = c.customer_id)a
where a.rk=1
order by a.customer_name

考察窗口函数