Re: Expressing SQL in relational algebra

neo55592_at_hotmail.com (Neo) wrote in message news:<4b45d3ad.0303280739.6213c846_at_posting.google.com>...
> How can I express "John likes Mary" using relational algrebra using
> only "John", "likes", "Mary", intersection operator(^) and grouping
> operators ("()")? The expression should not find "Mary likes John",
> "like John Mary", etc. Would the following expression be correct:
> (((John ^ John) ^ likes) ^ Mary)

I added you to my ignore list awhile ago so I don't ordinarily see your posts. I am forced to use deja tonight, so my ignore list is inactive.

One does not express axioms or postulates using an algebra. One uses an algebra to prove derivable facts from the axioms. One would express "John likes Mary" using a tuple. One would use the algebra to derive the tuples that satisfy some predicate such as "Who loves Mary?" Received on Wed Apr 02 2003 - 09:06:02 CEST