Re: Declaring super types

Keith H Duggar wrote:

>On Apr 25, 3:02 pm, r..._at_raampje.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.

>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.
>
>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.

>KHD

-- 
Reinier