Re: On Formal IS-A definition
Date: Wed, 05 May 2010 06:52:25 -0300
Message-ID: <4be13fe3$0$12440$9a566e8b_at_news.aliant.net>
David BL wrote:
> On May 5, 2:49 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
>>David BL wrote:
>>
>>>On May 5, 4:13 am, Tegiri Nenashi <tegirinena..._at_gmail.com> wrote:
>>
>>>>On May 3, 5:04 pm, paul c <toledobythe..._at_oohay.ac> wrote:
>>
>>>>>... and never bring up,
>>>>>say, just what the Information Principle really means.
>>
>>>>..."the entire information content of the database is represented in
>>>>one and only one way. Namely as explicit values in column positions
>>>>(attributes) and rows in relations (tuples)."?
>>
>>>>It is obsolete.
>>
>>>>Seriously, I think it is aimed at cowboys who try to invent new data
>>>>management systems without studying what relational model is about. It
>>>>is about attributes and tuples because both relational calculus, and
>>>>algebra explicitly refer to relations structured this way. The
>>>>situation is similar to arithmetic where pupils learn how to add/
>>>>multiply numbers represented in a very specific notation. So the
>>>>arithmetic principle would say ..."the entire content of arithmetic is
>>>>represented in one and only one way. Namely as explicit sequences of
>>>>digits in numbers". Presumably arithmetic principle would prevent some
>>>
>>>>from reinventing roman numerals:-)
>>
>>>I agree. My own take on the Information Principle is that "one and
>>>only one way" is misleading because one generally has a choice of
>>>whether to use rich or simple domain types. One could represent the
>>>entire database value in a single relation containing a single tuple
>>>with a single attribute and be adhering to the Information
>>>Principle. In that sense it seems rather vacuous.
>>
>>I disagree that it's vacuous. Even in this situation, the principle
>>prohibits physical pointers in the logical structure.
>
>
> Excellent point.
>
> Actually I wonder if it could even be said to be too restrictive.
> What if I want the entire database value to have a nominative type?
> This allows me to give the database a formal semantic (i.e.
> interpretation) that's explicit. E.g. my entire database might
> represent a complex 3D shape.
>
> Just as nominative types are necessary for supporting the definition
> of structural types, I think structural types are useful for
> implementing user-defined nominal types. Nominative and structural
> types are both fundamental and it seems arbitrary to force one not the
> other to be "at the top" of the database.
>
> So I suggest the Information Principle should be:
>
> "the database records a value"
That would be vacuous because every database records a value regardless of logical data model. Received on Wed May 05 2010 - 11:52:25 CEST