Re: Interpretation of Relations

Joe Thurbon wrote:
> I'm very new to this databases game, and am not even sure I'm using the
> terminology in the right way. I'd like some feedback as to whether I'm
> even in the right ballpark. Most of my understanding of the terminology
> comes from reading this group, and the definitions on Wikipedia.
>
> I've been wrestling with the correct interpretation of a relation. I'm
> currently working under the assumption that a relation comprises both a
> Type (or header) and a Body (or values).
>
> I'm going to start small.
>
> Consider modelling a situation in which there are people, and they have
> eye colour. I'm going to define some very small domains, so that I can
> enumerate the facts that I believe a given relation represents. I'm
> sorry if the notation is non-standard, but here it is.
>
> Domain D_People = {Joe}
> Domain D_Hair = {Red, Blond}

I'm going to stop here...

This means that everyone I will ever want to model in this database can be identified by 'Joe'; no-one will ever have any other identity. The domain is a type, and defines (in some way) all the possible valid values for that type. So, your domain of people can only ever identify Joe.

Similarly, your domain D_Hair says that the only possible hair colours of interest, ever, will be Red and Blond. No-one who has black, white, grey, brown, or purple-with-pink-stripes hair will be of interest to this database (or, if they are, they'll have to be treated as either red or blond or both - or neither?).

So, I think you've over-minimized your domains. Einstein said "Make everything as simple as possible, but no simpler".

-- 
Jonathan Leffler                   #include <disclaimer.h>
Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com
Guardian of DBD::Informix v2005.02 -- http://dbi.perl.org/