Re: domain questionnaire

vadim Tropashko wrote:
> I've just read C.J.Dates shots against some competetive technologies
> posted at http://www.firstsql.com/dbdebunk but feel that concept of
> domain also escapes rigid definition. In order to prove or falsify
> it, I compiled short list of questions and ask "Are those from the
> same domain?". Please post your opinions.
>
> 1. coordinate x of a point in cartesian coordinates vs.
> angle theta in polar representation of point

You can think of Domains as abstract data types. And as ADTs these two can be one and the same ADT which just happens to have two ways of denoting the same value. But note that it would be crucial to have different denotations for the two forms. So you cannot write them both as (x,y) and (r,a) but, for instance, can write them as C(x,y) and P(r,a).

> 2. weight in lbs vs. weight in kgs

The crucial question is again how you are going to denote them. Are you going to write one kilogram as '1 kg' or just '1'. In the first case you can have 1 ADT describing them both, in the second case you need two.

> 3. US voice stationary phone numbers vs. fax numbers vs. cell numbers

This is a subtle one. Date allows subdomains (or subtypes, or subclasses, or whatever you wanna call them :-)). So, in some sense these all belong to the domain 'phone number' but are in the same time different subdomains. But these are not really good domains because domains should be static, or, to be more precise, just as dynamic as the rest of the database schema. And since phone numbers can change their role quite fast, this should not be modeled with (sub)domains.

-- 
  Jan Hidders