You have to take all MembershipCards and remove all the ones that are in Purchase:
πcardID(MembershipCard) - πcardID(Purchase)
سؤال
Suppose you have two tables:
Purchase(cardID,itemID)
MembershipCard(cardID,clientID,companyName)
How can I get all cardID's which were not used in a purchase using relational algebra?
It seems like division to me but I'm not sure
المحلول
You have to take all MembershipCards and remove all the ones that are in Purchase:
πcardID(MembershipCard) - πcardID(Purchase)