select distinct y.cid from orders x, orders y where y.pid = x.pid and x.aid = 'a06'; ~~~~~~~~~~~~以前沒見過同一個表 兩次
原來要實現(xiàn)的目的是： Get cid values of customers who order a PRodUCt for which an order is also placed by agent a06. ~~要害就是這個詞了 ps:幾個表 CUSTOMERS cid cname city discnt PRODUCT pid pname city quantity price AGENTS aid aname city percent ORDERS ordno month cid aid pid qty dollars
