Re: Latest version of glossary

dawn wrote:
> JOG wrote:
> > However if forced
> > I would probably refer to however the term "entity" as defined in E-R
> > modelling.
>
> What would that definition be?

I assumed that given the widespread use of E/R modelling there would indeed be a good, easily accessible definition of the term. A quick search on google has highlighted to me that this was no the case - my mistake. Undeterred I dug up the original paper. In "The Entity-Relationship Model-Toward a Unified View of Data", 1976, P. Chen., his definition is:

 "An entity is a 'thing' which can be distinctly identified. A specific person, company, or event is an example of an entity. "

Awful. Note the apologetic quotes around "thing" - he is neither introducing a new term or quoting someone, so its awful use of English. Digging further into the paper yielded:

"Let e denote an entity which exists in our minds. Entities are classified into different entity sets such as EMPLOYEE, PROJECT, and DEPARTMENT. There is a predicate associated with each entity set to test whether an entity belongs to it, For example, if we know an entity is in the entity set EMPLOYEE, then we know that it has the properties common to the other entities in the entity set EMPLOYEE. Among these properties is the afore-mentioned test predicate. "

2 things here struck me as important. First, Chen writes that entities only exist 'in our minds', and as such are arbitrary constructions - I agree with him that this is an important part of any definition.

Second, it appears that he ties the definition in very heavily in with an "entity set". From this he seems to be defining an entity as: "An element of an entity set". Good, clear and simple definition that ;)

This would of course passes the buck to defining what an entity set is - but I believe that's an easier task, especially with Chen's reference to test predicates and common properties of elements of those sets. Opinions?

Now I found another interesting contribution wtih Edward Yourdon, who describes E/R in his work Modern Structured Analysis, (Prentice Hall 1989) and defines the concept of Entity as having three properties:

1. Each representation of an entity can uniquely be identified
2. Each representation of an entity is playing an important role in the system it lives in. (it has to have a reason to be there)
3. Each representation of an entity can be described by one or more attributes (data-elements, like name, age, quantity)

Hope some of this can be of use, all best, Jim. Received on Sat Feb 25 2006 - 05:34:35 CET