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Re: Can FK be nullable/optional by design?

From: Bob Badour <bbadour_at_golden.net>
Date: Thu, 18 Dec 2003 13:20:00 -0500
Message-ID: <ZoSdnbxjfv19cnyiRVn-hA@golden.net>


"Tobes (Breath)" <tobin_dont_spam_me_at_breathemail.net> wrote in message news:brq3iu$5nfbc$1_at_ID-131901.news.uni-berlin.de...
>
> "Bob Badour" <bbadour_at_golden.net> wrote in message
> news:aoCdnbmkVbe0SUKiRVn-tA_at_golden.net...
> > Your list of "fundamentals" does not answer any of the questions "What
is
> > data?", "What does it mean to manage data?" or "What principles
facilitate
> > or guide effective data management?"
>
> In that case I'd be interested in learning some of these fundamentals. I
may
> have to take myself to the library...

Try to find a library with a copy of the ISO/IEC Standard Vocabularies for Information Technology. A friend drew my attention to an article in IEEE Compute called _The Great Term Robbery_ a few years ago; I found both that article and the standard vocabularies very informative with respect to "What is data?".

I have never found a succinct list of principles, and if anyone knows of one, I would love to see it. Codd's 12 Rules embody a lot of principles he did not name explicitly; although, logical identity, guaranteed access, physical and logical independence are all principles. Certainly, the principle of separating concerns applies to data management in several ways. As a general principle, one prefers to minimize, centralize and automate any need for highly specialized or arcane knowledge. One prefers to maximize the portability of one's data. One prefers to make easy things easy and to make likely errors difficult. One prefers to minimize the learning curve for casual users. etc.

> > Of the items in your list above, integrity and data types are
fundamental,
> > but your elaborations above are anything but fundamental.
> >
> > One can come up with any number of taxonomies for integrity
> > constraints--Chris Date has published enough of them in his career. The
> > taxonomy I find most enlightening is: All integrity constraints
constrain
> > variables. Integrity is fundamental because it is fundamental to the
> > manipulation function when managing data.
> >
> > A data type does not enforce business rules--the integrity function of
the
> > dbms does this. Data type is fundamental to computing and not only to
data
> > management. A data type comprises both a set of values and a set of
> > operations on those values. With respect to the relational model, Date
and
> > Darwen have observed that data types define what we can make statements
> > about, and relations make statements about them.
>
> Hmmm, I thought Data Types (including UDTs) did enforce business rules, by
> constraining the set of possible values that can be stored in a column
> constrained to that type.

Data types form part of the definition of some constraints, but the integrity function of the dbms enforces constraints. What you suggest above is similar to suggesting that legislation and street signs enforce traffic laws. Police officers and the judiciary enforce traffic laws.

> If a business rule dictates that data of a certain
> type must fall within a spefic range, for example, then by defining a type
> that imposes this constraint, the business rule could be enforced by the
> Data Type?

The type does not impose the constraint; the integrity function of the dbms imposes the constraint. The type merely describes the constraint. For a very long time, almost all constraints in commerical SQL dbmses were nothing more than comments. One was allowed to express them, but the integrity function of the dbms ignored them (if one can really claim an integrity function even exists in that situation). Received on Thu Dec 18 2003 - 12:20:00 CST

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