Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Hi,

"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message news:MPG.1d33059837b1037c9896d7_at_news.ntnu.no... [...]
> The book unfortunately does not define "entity", but says that entities
> are "referenced" by "labels". It unfortunately does not define "label"
> either.

That's truly unfortunate.

> However, it tries to clarify thing with examples, e.g. by noting
> that the fact
>
> Pluto is owned by Mickey
>
> is ambiguous, because we don't know what the labels Pluto and Mickey
> represent. In contrast,
>
> The dog Pluto is owned by the mouse Mickey
>
> and
>
> The planet Pluto is owned by the person Mickey
>
> are not ambiguous, because the "entity types" dog, mouse, planet and
> person are mentioned.

But would not it be much simpler just to use first-order logic language and say:

Pluto is_owned_by Mickey & is_dog(Pluto) & is_mouse(Mickey)

What additional power does using the notions of "entity type" and the "label" provide ?

> I understand this as the approximately the same
> thing as the data type / domain concept in The Third Manifesto. Dog is a
> domain, with a possible representation of the dog's name as a string. In
> Tutorial D, you can refer to the dog Pluto as DOG("Pluto"), which of
> course is not even comparable to PLANET("Pluto"), since they are values
> from different domains.

Precisely.

> Where ORM and TTM differs, is that ORM has a tacit assumption that only
> certain domains can be used as representations, i.e. the lexical object
> types / labels. In TTM, this is not so. You can define a domain POINT,
> possibly represented by a pair of numbers X and Y (in some unit), or a
> pair of numbers R and THETA (each domain can have more than one possible
> representation (possrep)). You can then define a domain LINE, with a
> possrep of two points. And so on.

I think that multiple "possible representations" is merely an attempt to inroduce the union type (available in languages from C to ML) into the RM. I do not think that the Celcius vs. F example is motivating enough for multiple p.r. (this stuff belongs to the presentation layer rather than to the database).

More useful example might be this:

In ML:

datatype money = cash of int | cheque of string * real;

val x = cash 150;
val y = cheque("Bank of Whatever", 150.0)

>
> ORM talks about entity type (e.g. temperature), label (e.g. 37) and
> reference mode (e.g. degrees Celcius). In TTM, this is corresponds to
> domain / data type and possrep, which includes both "label" and "mode".

Yeah, probably (since I am not sure what label and the r.m. are).

Thank you.

Still wondering what a "semantic domain" as well as a a "conceptual object type" (http://www.orm.net/pdf/ER96.pd­f, "Conceptual query language"), might be. Do you have any idea ?

> Jon
Received on Tue Jul 05 2005 - 02:17:43 CEST