Re: Latest version of glossary
Date: 25 Feb 2006 09:05:55 -0800
> 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. "
Glad you tracked that one down as I hadn't done so recently.
> 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:
Conceptual data modeling is equally imprecise. It would be great if he had some wonderful-sounding, clear definition for "entity" but I'm satisfied with it being a thing. I did a quick check at dictionary.com and the first def of "thing" is "An entity."
> "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',
definitely a red flag phrase
> and as such are arbitrary constructions - I
> agree with him that this is an important part of any definition.
You are an entity and I'm figuring you exist outside of my mind.
> 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 ;)
Perhaps we could define all nouns that way? person: an element of a person set
> 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.
of course ;-)
> 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
In describing entities within the context of conceptual modeling, it is worthwhile to get the notion of identity into the mix, even if it is outside of the definition. We can talk about an egg as an entity at the start of our modeling efforts, while not yet determining a means to uniquely identify each instance of that entity.
> 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)
I think that is where the "of interest" part of the ISO def comes in.
> 3. Each representation of an entity can be described by one or more
> attributes (data-elements, like name, age, quantity)
This, again, is descriptive rather than definitive, in my opinion.
I really appreciate having the name and date of the paper handy. Thanks, Jim. --dawn Received on Sat Feb 25 2006 - 18:05:55 CET