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.
~~要害就是这个词了
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