Re: Object-Role Modeling?

Marshall Spight wrote:
> Alfredo Novoa wrote:
>

>>> From what I have read of it, it seems that it would be the
>>>place to start,
>>
>>I prefer to start defining relvars. 

Many people seem to like to begin with ER diagrams. I happen to think that there's a reason for why they caught on as they did. And if you supply them with a formal semantics and a constraint and query language (which is easy to do and has in fact already been done by several researchers) then it qualifies as a database schema. So actually they *would* be beginning by designing a database schema.

> If we want to raise the level of abstraction, it is necessary
> to omit certain details in order to focus on other details.
> But all of the details of a table definition are important;
> you can't just skip over any of them.

Only implementation details like choosing primary keys or choosing whether or not to embed a one-to-many relatinship have to be skipped. All logically relevant details will still be in there.

> The might *possibly* be some opportunity (as I think Jan was
> suggesting in another thread) to abstract the relationships
> between tables, but I am sceptical.

You wouldn't be thinking about relationships between tables. The whole point is that you can model at a higher abstraction level in terms of entities, properties of entities and relationships between entities. How you map that to tables and relationships between these tables, whether you split entities into several tables or merge several into one, how you model the ISA relationships, whether you embed the one-to-many relationship into the entity, those are all implementation issues.

• Jan Hidders