Re: I think my book may be wrong about cardinality, but I'm not sure
From: paul c <toledobythesea_at_oohay.ac>
Date: Tue, 24 Jul 2007 22:03:16 GMT
Message-ID: <EGupi.4117$rX4.2472_at_pd7urf2no>
>
>
> No. An N-degree table implements an N-ary relationship between attributes -
> attributes which may well identify other entities. Do not assume
> relationship = foreign key. A foreign key is just one type of constraint
> (not necessarily the only one) for enforcing referential integrity in RM.
>
Date: Tue, 24 Jul 2007 22:03:16 GMT
Message-ID: <EGupi.4117$rX4.2472_at_pd7urf2no>
David Portas 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?
p Received on Wed Jul 25 2007 - 00:03:16 CEST