Re: Smart Database Tricks

David Cressey schreef:
> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
> news:467bbfa9$0$4321$9a566e8b_at_news.aliant.net...
> > Jan Hidders wrote:
> > > On 22 jun, 01:36, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> > >
> > >>Is an attribute a predicate variable of some sort?
> > >
> > > As in "the x in predicate P(x, y) "? Yes, I'd say that is the same
> > > thing.
> >
> > I thought so. When one writes a query and requests certain attributes,
> > would those requested attributes be free variables in a logic sense?
> >
> > When one uses an attribute in a restrict condition or join condition,
> > would that attribute be a bound variable in a logic sense?
>
> I don't know what the term "free variable" means in the world of logic. I
> do know what it means in the context of Lisp programming. (Or, at least, I
> used to know). So far, the use of "free variable" in these discussions
> seems to be consistent with the Lisp usage.

There's different meanings of the term "bound". The notion of free variable is simply defined as "not bound by a quantifiers". So in the formula "forall x : p(x,y)" the variable x is bound but y is free. But at the same time it is sometimes said that "y is bound to the second position of predicate p". So there is "bound by a quantifier" and "bound to a position in a predicate" and only the first is the opposite of the usual notion of "free". Hopefully that clears it up a bit.

• Jan Hidders