Re: Very simple question to relational theorists.

On Thursday, September 6, 2012 2:00:59 AM UTC-7, Evgeniy Grigoriev wrote:
> ...Please, answer me before further discussion - will I change predicate when I rename attribute of some relation?...

I have been using "predicate" as a mapping from a world situation and a truth value. (Just as in predicate logic.) RENAME of a relation corresponds to changing all free occurrences in its predicate.

So if the wff R(A,B,...) denotes the predicate for R then the wff for R RENAME A TO N is R(N,B,...). If the wff S(X,Y,...,A,...) denotes the predicate for S then the wff for (R JOIN S) RENAME A TO N is R(N,B,...) AND S(X,Y,...,N,...).

Whatever you do with this value you know that for that state it was the extension of the predicate of that expression.

(A base relation variables has its own predicate. Its value is the extension of its predicate. If you assign an expression's value to a relation variable then that value is assumed to be the extension of the relation variable's predicate in the new state. But that value is still for that earlier state the extension of the predicate of that expression.)

philip Received on Fri Sep 07 2012 - 12:11:39 CEST