Re: I think my book may be wrong about cardinality, but I'm not sure
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 24 Jul 2007 19:12:27 -0300
Message-ID: <46a6791f$0$8850$9a566e8b_at_news.aliant.net>
>
> Just a side question, since I see the term "N-ary relationship" quite
> often. If a relation has N attributes, doesn't it implement (2**N)-1 or
> 2**N relationships or thereabouts? Or is that what the term means and
> I've been misunderstanding it all this time?
Date: Tue, 24 Jul 2007 19:12:27 -0300
Message-ID: <46a6791f$0$8850$9a566e8b_at_news.aliant.net>
paul c wrote:
>> "beginner16" <kaja_love160_at_yahoo.com> wrote in message >> news:1185301676.680771.29780_at_b79g2000hse.googlegroups.com... >> >> >>> 2) >>> Relationship between two entities is called binary connection or >>> second degree relationship. But connection can exists between more >>> than just two entities. Level of connection is determined by the >>> number of different entity types that exist in a connection. >>> >>> Now as far as relational DB goes, don't tables have only binary >>> connections ( second degree relationship )? >>> >> >> >> No. An N-degree table implements an N-ary relationship between >> attributes - attributes which may well identify other entities. Do not >> assume relationship = foreign key. A foreign key is just one type of >> constraint (not necessarily the only one) for enforcing referential >> integrity in RM. >>
>
> Just a side question, since I see the term "N-ary relationship" quite
> often. If a relation has N attributes, doesn't it implement (2**N)-1 or
> 2**N relationships or thereabouts? Or is that what the term means and
> I've been misunderstanding it all this time?
They each describe a single relationship of a particular arity. Received on Wed Jul 25 2007 - 00:12:27 CEST