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
