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Re: Entity vs. Table

From: ben brugman <ben_at_niethier.nl>
Date: Mon, 14 Jun 2004 14:13:32 +0200
Message-ID: <40cd9655$0$4922$4d4ebb8e@read.news.nl.uu.net>


On most I do agree with you.

We have some differences.
For me a fysical model (a term I try to avoid) is the model which represents the fysical implementation. The closer the model describes of what it is, the more detailed (and sometimes better) the model is. But I prefere to use the fysical implementation, because I know what the fysical implementation is.

But it seems that you use the 'fysical model' as a model which is made from the logical model before implementation is made.

The fysical implementation, normaly does have indexes, errorchecking bits and is often implemented on a Raid. Therefore the implementation contains redundancy. The fysical implementation therefore is not 3NF, although it often is an implementation of a logical model which should be 3NF.

(By the way, indexes (which can be rebuild) are definitely redundant information).

ben

"Alan" <alan_at_erols.com> wrote in message news:2ittqgFqitf3U1_at_uni-berlin.de...
>
> "ben brugman" <ben_at_niethier.nl> wrote in message
> news:40c9a2db$0$4927$4d4ebb8e_at_read.news.nl.uu.net...
> > I do not think a term can be wrong, it's use can be wrong.
> > (But before that there has to be agreement on what the term
> > means, defines, or represents).
> > But if : "But everyone just uses the term "entity" to
> > mean "entity set", and tuple, ", they must be right because
> > there is nobody to oppose them.
> >
> > >
> > > Only if you want to implement 3NF correctly, so, yes. In some cases
you
> > have
> > > some choices, in other cases there is only one "best" way.
> > >
> > 3NF dictates no redundancy. Indexes are redundancy.
>
> No they are not. Indexes have nothing to do with 3NF. They are strictly an
> implementation detail.
>
> So
> > most implementations are not 3NF.
> > The 3NF term belongs to the domain of the concept, or logical model,
> > not to the implementation.
> For complex situations there often are loads
> > of 'best' ways, for the logical model as wel as the implementation.
> >
> > But I do understand your desire to keep the implementation as
> > close as possible to the logical model. (I do not see this as an
> > requirement).
> >
>
> It's not a requirement, but it is a starting point. If you start with a
3NF
> physical model, you can denormalize for the sake of performance.
>
> > >
> > > There are definite rules to convert an ERD (your logical design) into
> > tables
> > > (the physical implementation) to achieve 3NF, but you obviously don't
> know
> > > about them.
> > >
> > Sorry, here the language problem creeps up. With rules I meant 'binding
> > rules',
> > not rules as 'guidelines'.
>
> Again, there are definite rules to get to 3NF, and some of these rules
> include a choice.
>
> >
> > >
> > > I made that up. Don't believe everything you read, especially on the
> > > internet.
> > >
> > There is no problem with you making up an example. Examples (real or
> madeup)
> > can be used perfectly to illustrate something.
> > Except that this example did not illustrate your point.
> >
> > >
> > > An arificial key. So what? Sometimes you need one. There are lengthy
> > > arguments on both sides as to whether or not to use a natural key (if
> > > available), or to always use an artificial key.
> > >
> > Do you mean an artificial key. (Like ISBN, social security number)
> > This is by no mean meaningless (to me and most others).
> > Or
> > Do you mean a surrogate key. (Totally internal not to be communicated to
> > the outside world).
> > (The surrogate key if viewed on it's own does not hold information).
>
> Surrogate key. I use the terms "artificial" and "natural". In my
> definition, SSN or ISBN is a "natural" key, because it has meaning. An
> artificial key, would be your surrogate key. I'm not dogmatic about it,
> though. It just has more meaning to me. I've heard both sets of terms
used.
>
> >
> > >
> > > > Before computers, information that was written down (or kept in any
> > other
> > > > form), there was no real distinction between 'the holding of
> > information'
> > > > and
> > > > the 'presentation' of information.
> > >
> > >
> > > Not true, actually. Before the general population knew how to read,
the
> > > "priests" of knowledge routinely distorted data to enhance their own
> > power.
> > > Enter organized religeon (just ask Galileo) - but that's a real big
can
> of
> > > worms.
> > >
> > What is your point here. I allready made that exception :
> > >
> > > (Except of encription to make the
> > > > information
> > > > only accesable for the 'intended' users).
> > > >
> >
>
> Semantic argument. "Accesable for the 'intended' users (sic)" implies
taking
> clear data and obfuscating it- at least to me.
>
> > >
> > > In this forum, it is important to distinguish between the business
> model,
> > > the logical model (there is much overlap, though), and the physical
> model.
> > > Entities belong to the first two, tables to the last. In this case, it
> is
> > > important to distinguish what is right from what really happens to
avoid
> > the
> > > confusion you wrote about.
> > >
> > I do agree with you.
> > It would be nice to have definitions for these terms which are usable
and
> > understandable by most.
> >
> > A TRY :
> > Physical datamodel is a model of the
> > database implementation ? (The databasemodel).
> > (Can contain redundancy does not have to be like the
> > logical datamodel, but MUST implement the logical model).
>
> Yes and no. There should be no redundancy, or it is not a propely
> implemented 3NF relational database. Otherwise, yes.
>
> >
> > Logical datamodel is describing the data in at least
> > 3NF (with no redundancy) and
> > should be implementing/describing the business model.
>
> Yes.
>
> > An ERD is often used to describe part of the logical datamodel.
> >
>
> A logical data model follows after the ERD, so I would say no to this
part.
>
>
> > Business model is a description of the problem / solution / a desired
> > solution / a desired description of the 'real' world.
> > (There is no requirement how the business model is described.
> > Sometimes an ERD is used for a part of the business model).
>
> Yes, with one small change. An ERD is a conceptual business model of the
> data and the realtionships(associations) among the data.
>
> >
> >
> > just my 2 cents
> > ben brugman
> >
> > >
> >
> >
>
>
Received on Mon Jun 14 2004 - 07:13:32 CDT

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