Re: What is a Relationship !?

ranger wrote:

> 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.

Hi,

It depends on what book you read. Try Fundamentals of Entity Relationship Modeling by Bernhard Thalheim.

An entity may be identified by attributes, or in less frequent situations it may be a combination of attributes and other relationships in which it participates .

A relationship is a subset of the cartesian product over entity types or relationships. Thus it needs no attributes to be identifiable.

Let me give you a hypothetical example:

• if you have the abstract entity type Person, with entity subtypes Person-With-SSN, Person-Without-SSN. And let's say Person-With-SSN are identified by SSN (as candidate key), Person-Without-SSN is identified by an alternative identification mechanism (say: FullName, DateOfBirth, PlaceOfBirth, IssuerOfBirthCertificate, BirthCertificateNumber)

Now you have the relationship Marriage. In relational model proper you'd have to come up with the collumns for Marriage, which poses bothe theoretical and practical problems.

In HERM (the most advanced form of E/R) you wouldn't need to. Marriage is in abstract just a subset of the cartesian product Person x Person. Marriage should not depend on the internal details of each subtype of person. Received on Fri Nov 21 2003 - 15:44:48 CET