Re: Proposal: 6NF
Date: 18 Oct 2006 15:15:49 -0700
Message-ID: <1161209749.198397.239990_at_b28g2000cwb.googlegroups.com>
J M Davitt wrote:
> paul c wrote:
> > David Cressey wrote:
> >
> >> ...
> >>
> >>> PMFJI, when it comes to a dbms engine, I don't know the difference
> >>> between a type and a domain.
> >>>
> >> In the context of a dbms engine, a type would be the built in datatypes
> >> that the engine supports, like INTGER, DECIMAL, CHAR, and DATE, and also
> >> the builtin functions and operations, like "+" or "weekday(x)".
> >>
> >> A domain would be what you get when you say CREATE DOMAIN. It's a
> >> set, but
> >> it has no functions and operations other than the ones it inherits
> >> from the
> >> data type it is based on.
> >>
> >> Does this make sense?
> >>
> >
> > Ièd say YOU make sense, but IT doesnèt. However, if the difference you
> > mention is typical usage, I guess Ièll have to accept it. I vaguely
> > remember using something like CREATE DOMAIN in SQL, perhaps that is
> > exactly what I used, and being disappointed - I seemed no more than a
> > name aliasing operator.
> >
> > p
>
> There's lots of potential confusion, here. The D+D crowd
> can be expected to say, "Wait! The only types that should
> be built-into a dbms are boolean, tuple, and relation."
> Their view is that *users* should be able to specify what
> integers, rationals, strings, dates, gender, countries,
> currencies, etc., etc. the database should be able to handle.
> And, by handle, I mean represent, store, and operate upon.
>
> In that regard - and, acknowledging that dbmses are limited
> to what computers are capable of representing - types and
> domains are the same thing. Types and domains are *not*
> what SQL says you get after CREATE DOMAIN, and operations
> must (generally) be defined independently of types --
> although, of course, types must be extant before operations.
>
> The "what computers are capable of representing" remark is
> important: while we might describe the set of, let's say,
> "extended integers" as "zero and naturals and negative naturals
> and infinity and negative infinity," it's obviously impossible
> to represent many of those values using computers.
A nit, perhaps, but which values would those be that we cannot
represent with computers?
--dawn
Received on Thu Oct 19 2006 - 00:15:49 CEST