Re: Plural or singular table names

"Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message news:bj73hq$133e$1_at_gazette.almaden.ibm.com...
> Plainly I don't agree with you that constraints are part of the type specification.

I guess I should clarify that (obviously?) I mean that constraints are not part of the type specification of tuple and relation types.

Also, I'm obviously aware that sub types are specified by a constraint on a super type. With type inheritance, the rule that values must be of the same to possibly be equal is replaced by the rule that as long as one type is a subtype of the other then their values can possibly be equal.

TTM formulates type inheritance directly only for scalar types. Tuple and relation type inheritance follows from any subtyping of their attributes, not by any (other) constraints that they might have.

So if you have two relvarsA and B. Both with type

    RELATION{ H }
where

    H = {<X, INTEGER>, <Y, INTEGER>, <Z INTEGER>}

and A has the key {X, Y, Z} but B has the key {X, Y}, then *even if* you considered A to be of a different type of B (I and D&D don't), then you might consider that type of B would be a subtype of A, and so the value of A will equal the value of B if they contain the same set of tuples of type TUPLE{H}.

But as I said, I don't believe that type inheritance needs to be defined separately for relations. See IM Prescription 25: Tuple/relation Variables with Inheritance for more details.

Constaints are not part of the type specficition of relations and tuples.

Regards
Paul Vernon
Business Intelligence, IBM Global Services Received on Thu Sep 04 2003 - 15:13:58 CEST