Re: Lucid statement of the MV vs RM position?

dawn wrote:
>[chop]
> Not entities. Entities are things. It is a person, place, thing or
> event. Person, place, and event (e.g. transaction) are easy enough to
> identify. For things, if you cannot hit it with a stick or print it
> out on paper, then think twice about whether it is an entity or a
> property of an entity.

No, this is way off base - with ERM Chen wrote extensively about associative entities for example, which are _exactly_ the sort of thing that cannot be hit with a stick. But I don't want to get sidetracked by this so hey, if the colour green as an abstract entity is too out there, fair enough. Just substitute in something more physical into those original statements - fruits for the colours for example, or perhaps academic papers:

paper( paper1, {Barney, Oscar} ) &&
paper( paper2, {Barney}) &&
paper( paper3, {Oscar})

person( Barney, {paper1, paper2} ) &&
person( Oscar, {paper1, paper3} )

Everything here is clearly an entity. Both are strong. Both representations come from those logically identical propositions - so which is the right one?

Instinct might be to go for the first - papers have authors after all. But my application's first job is to produce academic resumes - hence the second is the more appropriate one for that 'interface'. Whichever is picked, the choice enforces some form of arbitrary hierarchy. Unless I encode both - is that preferable?

>
> > Which is correct? Neither
> > and both,
>
> The first, not the second.
>
> > because they are artifices. Who knows which will be
> > appropriate to the user?
>
> The systems analyst better be able to give it a good shot. If they
> cannot tell whether green-ness is an entity or a property of an entity
> wrt to the organization's requirements, I would be very surprised.
>
> > An XML or MV style prejudges that decision,
>
> No, it makes a distinction that those modeling with the RM do not make.
> I am an entity and my shoe size is a property of me. It can change.
> It could even go away if my feet were amputated (sheesh, apologies for
> the example), but only as long as I am an entity an organization cares
> about would that property be relevant. If I am not of interest, then,
> by definition, my feet are not of interest either. In other words, if
> you are interested in shoe size, you are interested in me because my
> shoe size is a property of me.

Yes, a weak entity. This is indeed a good example of something that does fit hierarchically, but it *doesn't* negate the fact that there are many examples that don't at all. Using colours as universals was a poor example on my part as I feel its given rise to this red-herring.

>
> [I realize that if a company is doing a shoe size survey and has no
> interest in collecting other data about people, then the shoe size
> might be a property of a survey or some other entity.]
>
> > and with large shared data, who's future use is unpredictable, it seems
> > essential to me to avoid that interface-prison.
>
> I disagree, as you have likely guessed. Yes, there are new
> requirements regularly that prompt one change or other, but I would
> dare say that there are more changes that would push a property like
> color to go from single to multi-valued, prompting a schema redesign
> (new table) than there are if you make color a multivalued attribute to
> start with.

But, two wrongs don't make a right. Cardinality change is something i've mentioned before as an interesting area in schema change as a whole - but it does not mean the theoretical solution should be to imprison the user into a certain view of the data. all best, J.

>
> > With RDBMS, despite thei 3VL issues, there is no prejudice as to this
> > choice - both sets of propositions, 1 and 2, encode down to the same
> > things - just as they should being logically identical.
> >
> > > I reserve the right to change my mind on that, however. Cheers! --dawn
> >
> > I'd reckon that this is the most important quality anyone can have in
> > life. That and a high tolerance for alcohol.
>
> I have the first even if I argue until I fully understand another
> position, but don't let me drink more than one martini. --dawn
Received on Wed May 03 2006 - 05:41:10 CEST