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?

The constraint 'A < B' suggests to me that there is 'something' of A in B or the other way round. In this case it is easy to engage some isolation. The base table (say W) should just contain A and C, C being a positive number. X becomes a view with A = W.A and B = W.A + W.C

That this isolation is possible in this example should not be taken to mean that it is in all cases you have in mind, though.

My 2 Eurocents. Received on Mon Jun 06 2005 - 20:22:29 CEST