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假设您有两个表:
购买(cardID,itemID)
MembershipCard(cardID,clientID,companyName)
如何使用关系代数获取所有未在购买中使用的 cardID?
对我来说这似乎是分裂,但我不确定
您必须拿走所有会员卡并删除购买中的所有会员卡:
π cardID (MembershipCard) - π cardID (Purchase)