一条SQL语句-数据库专栏,SQL Server

以前没见过。。。
数据库这一块这是大啊。。。

select distinct y.cid from orders x, orders y where y.pid = x.pid and x.aid = a06;
                                      ~~~~~~~~~~~~以前没见过同一个表 两次

原来要实现的目的是：
get cid values of customers who order a product for which an order is also placed by agent a06.
                                                                                                           ~~关键就是这个词了
ps:几个表
customers cid cname city discnt
product pid pname city quantity price
agents  aid aname city percent
orders ordno month cid aid pid qty dollars