Declaring super types

#21

Reinier Post wrote:

[...]

Quote:

Yes, but you can rename it to axisX, although that isn't a very
logical name unless you know you're going to create an Ellipse
as a ass, and even then it might as well be called axisY.

^--- not sure what happened there. I meant to write 'subtype'

--
Reinier

#22

Keith H Duggar wrote:

Quote:

On Apr 25, 3:02 pm, r... (AT) raampje (DOT) lan (Reinier Post) wrote:
Tegiri Nenashi wrote:
Second, why would I add redundant attributes to a Circle? If the idea
is to make both relations to have the same set of attributes, then we
go back to the previous paragraph: I'm interested to see a convincing
example of two relations with different sets of attributes that fits
your definition.

Person: first name, last name, date of birth
Citizen: first name, last name, date of birth, country of citizenship

I've done some student instructions with that textbook and I still
use the same ER modelling technique for myself; I've noticed that
this is-a comes up pretty often, and that it is helpful, i.e. many

Does "is-a" come up because it follows naturally from the design
process? Or does it come up the same way that Object Oriented comes
up these days in programming discussions ie being shoe-horned into
the conversation whether needed or not?

It comes up naturally in that when you have it in your ER modelling
language, you find applicattions in most models.

Quote:

How is this is-a concept "helpful" as you claim? For example, I
can't imagine myself every creating a database with the separate
Person and Citizen tables above.

modelling errors I see can be explained in terms of "is-a being overlooked"
or "is-a being modeled incorrectly". It is also fairly common in tools.

To repeating myself: it is helpful in that when you have it in your
modelling repertoire you can not only construct more concise models
but you also recognize certain modelling errors in models created
by modelers who don't, that can be explained in terms of it.
E.g. is-a vs. has-a confusion.

However this is no more than anecdotal evidence for its benefit.
Modelers may also introduce certain errors into models due to
"is a", perhaps to lack of understanding, that they wouldn't be
so likely to commit without it. I don't know.

Quote:

And what happens if we simply banish "is-a" from our thinking and
vocabulary entirely? Are those modelling errors eliminated? What
do we lose by sacrificing this hierarchical notion?

We lose some syntactic sugar.

Quote:

KHD

--
Reinier

#23

On Apr 26, 3:25*pm, r... (AT) raampje (DOT) lan (Reinier Post) wrote:

Quote:

E.g. is-a vs. has-a confusion.

You imply that there is a standard has-a definition expressed again
purely by relational means? Any example?

#24

On 20 apr, 04:13, David BL <davi... (AT) iinet (DOT) net.au> wrote:

Quote:

So they can
*explain* what implicit coercions should be allowed and what aren't.

My understanding was that they opposed to coercions alltogether.

#25

On May 7, 1:28 am, Erwin <e.sm... (AT) myonline (DOT) be> wrote:

Quote:

On 20 apr, 04:13, David BL <davi... (AT) iinet (DOT) net.au> wrote:

So they can
*explain* what implicit coercions should be allowed and what aren't.

My understanding was that they opposed to coercions alltogether.

I meant "coercion" in the context of a statically typed imperative
language.

e : ELLIPSE;
e := CIRCLE( POINT(0,0), 1);

Coercion has to do with static type analysis, and is not meant to
suggest for example that a variable can be assigned a different value
to what is evaluated in the right hand expression.

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Declaring super types

comp.databases.theory comp.databases.theory

Re: Declaring super types - 04-26-2010 , 05:12 PM

Re: Declaring super types - 04-26-2010 , 05:25 PM

Re: Declaring super types - 04-26-2010 , 06:26 PM

Re: Declaring super types - 05-06-2010 , 12:28 PM

Re: Declaring super types - 05-06-2010 , 06:51 PM