Re: domain questionnaire

On 23 Feb 2001 13:30:25 GMT, hidders_at_REMOVE.THIS.win.tue.nl (Jan Hidders) wrote:
>> ORM being Object Relational Model?
>
>Er, sort of, it's Object Role Modelling. If you want to know more:
> www.orm.net
> www.inconcept.com

Oh, well, I may have glanced at this before, but obviously it didn't much stay with me. And here they are, having a conference right next door in Anaheim next month. Yet Another Methodology ...

>> I think it may be impossible to do
>> this correctly and still see it as a dialect of relational -- and if
>> E/R is something other than relational, I've missed it.
>
>I'm not sure I understand this. Are you saying that there is no
>difference between the ER model and the relational model?

I've never been able to make any use of the distinction between logical and physical data models -- so I guess I tend to disregard what others may value highly.

>And do you
>think that it is not possible to have an object-relational model?

All I meant to say is that an effective combined model might not preserve all of the current dogma of either, might not be a DIALECT of relational, so much as a replacement or superset.

>> How can you compare two objects, if you don't have their identities
>> separate before hand?
>
>By having some way to refer to them. That is not the same as knowing
>how to identify them, i.e., determine if two references refer to the
>same object. You know, "morning star", et cetera.

We're getting very Lewis Carroll here, "What do you call the name of the song?" she asked. There are many, many subtle complexities. How would you model the morning star issue in relational terms? <g>

>Sure, you can, and as far as I know most formal object-based data
>models do. But the question is if you should introduce a design
>principle that says something like "for every class you have to have
>some predicate that is expressed in terms of the schema, that decides
>when two objects in this classe are one and the same object".

Yup, that's the question, or at least a major question. As you say, most object systems do this, ... but most relational systems have not, at least it wasn't within the standard.

Once you allow for identity, you may need to reify other things that relational has always treated as metadata -- timestamps and ordering come to mind.

Joshua Stern
JRStern_at_gte.net Received on Fri Feb 23 2001 - 19:40:46 CET