Re: Smart Database Tricks

On 21 jun, 21:17, vldm10 <vld..._at_yahoo.com> wrote:
>
> On Jun 18, 7:41 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> >
> > On 18 jun, 20:59, vldm10 <vld..._at_yahoo.com> wrote:
> > >
>
> This confuse me. For example you asked me " How do you define the
> state of an entity?".
> I have this definition on my web. It is given in 5.7 .

I do not think it is precise enough. For example, if I have an entity set E1 and enity set E2 and a relationship R between E1 and E2. Assume the pair (e1, e2) is removed from R. Have the states of e1 and e2 now changed, or is this only a change of the state of the pair in R?

> > So,
> > since I don't understand your terminology well enough it is simply not
> > possible for me to continue this discussion in a meaningful way.
>
> Before we complete the discussion about the conceptual level I have
> one question.
> Here in example1 I gave the identifier of an entity Car. It is about
> VIN = "vehicle identification number".
> The identifier is not local for Car entity. It is global. Let me say
> we have here global lawful information, which is completely accepted
> as successful solution in the reality and in every country. You state
> that "...adding such artificial implementation concepts are usually a
> bad idea..." Can you explain why VIN is a bad idea? Otherwise I can't
> accept your position about the identifiers.

If VIN is a natural notion that already appeared in your Universe of Discourse, then there is nothing wrong with it. It is the artificial addition of such concepts that I object to. And from your definitions it is not clear whether you allow, or indeed advocate, such artifical additions or not.

> > > Here the CarKey is the Identifier of the state of the entity Car and
> > > this is the
> > > only column of the table which has unique values. So the attribute
> > > CarKey
> > > is the primary key. Car ID is an Identifier of the entity Car. We use
> > > VIN values
> > > for this attribute. Maker and Color are the attributes of the entity
> > > Car.
> > > "999999" means that corresponding data is current.
> > > Here Datefrom and Dateto are strictly related to attribute Color (not
> > > to entity car).
> > > Datefrom and Dateto are not the attributes. They are a part of our
> > > actual
> > > knowledge about the attribute color.
> > > (so here the "The information principle" is not appropriate, because
> > > the entire
> > > information content is not represented as values in attribute
> > > positions, i.e.
> > > here we also represent knowledge about attribute Color )
>
> > That depends on what you call an attribute.
>
> I am using my definition of the attribute. This definition is on my
> web site under 5.2.

Sure, but the information principle is formulated for the relational model, which uses another definition. If you change the meaning of the words it can no longer be called the information principle.

> > If this is a relation then
> > all columns are attributes and since you have represented all the
>
> Can you please, tell me what is the definition of the attribute in RM.

It's a column of a relation. Conceptually it represent the role in the predicate that is represented by the relation.

• Jan Hidders