Re: What is a Relationship !?

The fact that a relational model does not allow a distinction between entities and relationships is not evidence that there isn't a difference. According to the online class notes from Database Systems and Structures at Simon Fraser University
(http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node2.html#SECTI

ON00110000000000000000):

An entity is an object that exists and is distinguishable from other objects. For instance, John Harris with S.I.N. 890-12-3456 is an entity, as he can be uniquely identified as one particular person in the universe.

This is a pretty common notion. An entity exists in the real world and has a unique identifier. A relationship on the other hand is some subset of the power set of entities, or as DSS defines it: A relationship is an association between several entities.

Typically, relationships are identified by the identifiers of the entities that fill key roles in the relationship, not by any unique identifier of their own. However, relationships do often have additional attributes over and above their member attributes.

"Bob Badour" <bbadour_at_golden.net> wrote in message news:n-GdnZE11aGSZiaiRVn-uA_at_golden.net...
> "ranger" <ranger101_at_hotmail.co.il> wrote in message
> news:d6cdf6f8.0311191432.58dabe3c_at_posting.google.com...
> > Hello there,
> >
> > I feel like most of the papers I have read about E-R seem to be avoiding
> > really dealing with the questions in particular relationships
> > what to they mean a relationship is the association of entities ???
> >
> > Is a relationship the fact that they are related e.g. book on a shelf
> > meaning that the place of a book is on the shelf
> > i.e. something that he is like the height of a man ???
> >
> > Or is a relationship something by itself that is the relationship
> > e.g. component in recipe meaning a physical something that is
> > the same as the component and is in the recipe ???
> >
> > Please help ! the words are starting to lose their meanings.
>
> The distinction between an entity and a relationship is arbitrary. The
> relational model represents either as a relation.
>
>
Received on Thu Nov 20 2003 - 19:40:52 CET