SQL新手求教一个关于数据库的问题，着急！在线等！各位大牛们，感激不尽！

Question

实习生，TeamLeader给了那么一个数据库题目：
给了如下两张独立的表：
Person

Orders

要求从这两张表中选出以下结果：

ItemCount是购买的总数！
求救！在线等！

Answer 1

 select t.Id_p,t.LastName,COUNT(OrderNo) as ItemCount from 
(
select t1.OrderNo,ISNULL(t1.Id_p,t2.Id_p) as Id_p,isnull(t2.LastName,'Unknown') as LastName from Orders t1
full join  Person t2
on t1.Id_p = t2.Id_p
) t group by t.Id_p,t.LastName