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# Re: Relational symmetric difference is well defined

From: Jan Hidders <hidders_at_gmail.com>
Date: Fri, 15 Jun 2007 05:21:01 -0700

On 1 jun, 03:40, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
>
> > On May 30, 8:52 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
> >> Can you clarify the difference between set containment join and set
> >> equality join? The inverse of join is much on my mind these days.
>
> > Set equality join
>
> > A(x,y)/=B(y,z) is {(x,z)| {y|A(x,y)}={y|A(y,z)} }
>
> > Set containment join
>
> > A(x,y)/=B(y,z) is {(x,z)| {y|A(x,y)}>{y|A(y,z)} }
>
> > where the ">" is "subset of".
>
> The above formulas obviously are no longer first-order expressions.
> Along with the increased expressive power (e.g. it's trivial to define a
> powerset), you will reap the usual drawbacks of the higher order logic.

This was perhaps already clear, but it is the *formulation* of the semantics which is not first-order. The semantics themselves are clearly first order since they can be defined in first order logic or the flat relational algebra.

• Jan Hidders
Received on Fri Jun 15 2007 - 07:21:01 CDT

Original text of this message

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