Re: Translating constraints to RM Terms

Kenneth Downs wrote:
> I'm wondering if some RM theorist might express something for me.
>
> Consider table X with columns A and B. There is a constraint that A must be
> less than B. While we discuss such constraints every day, I realize I do
> not know how to express such constraints in relational terms.
>
> For instance, A is actually a domain, as is B, but the constraint A < B must
> in fact be part of the very definition of the domain A, and here we are
> defining one domain in terms of another. I realized I have not seen this
> disccussed in the year or so I have been a regular here. Is defining one
> domain in terms of another allowed and considered trivial, no big deal? Is
> it actually not allowed and this is some SQL alteration of true RM?
>

I don't think it is neither necessary nor allowed. The column (or attribute) "A" is a different entity from the domain "A". The column "A" draws it's values from the domain "A" (ditto for B). The contstraint could be classified as a table (or relational) constraint and it is easy to express in SQL.

Best Regards
Lauri Pietarinen Received on Sun Jun 05 2005 - 21:16:14 CEST