Re: [LIU Comp Sci] Need tutoring on Relational Calculus

On Sunday, December 21, 2014 5:26:19 AM UTC-8, Eric wrote:
> On 2014-12-21, Ruben Safir wrote:
> > Eric wrote:
> >> On 2014-12-18, ruben safir wrote:
> >>> ...
> 8>< ----- many things snipped
>
> A lot of compdb's responses are things I might have said, though probably
> in a different way.

Tactfully?

> > But it still has degrees of freedom. Everything is bound to something.
> > Everything is free within its bounds of degrees of freedom.
>
> Good point.

In another post I suggested his comment is vague, confused and irrelevant.

> Though the "universe" of a database is only those things
> that it is designed to store, not the star dust and dark matter. Free
> and bound here are merely relative to that "universe of discourse".

I can think of an interpretation of what you are writing where this statement is justifiable, but still not reasonable. This is doing Ruben an injustice in validating vagueness. Considering that the definitions in question are about expressions (trees or sequences of string or tokens) and are as precise and as prosaic as "An integer divisible by 2 is said to be *even*. Other integers are said to be *odd*".

> What other quantifiers would you have? And what's wrong with universal?
> (∀ t)(F) just says that F is a fact, true for all values of the variable t.

(∀ t)(F) doesn't say anything unless it has no free variables. In that case it says that an expression is TRUE if it is like F but with F's free occurrences of "t" replaced by an expression denoting a value from the domain of t. Alternatively, it says that if you make such an expression for every value from the domain of "t" then (the expression that is) their conjunction (ANDing) is TRUE.

I'm sure you know this. But again, an injustice. The fact that an informal "just" is inadequate is why the text is full of a bunch of (necessary) definitions. (And observe the effort and care needed to be concise and precise.)

philip Received on Mon Dec 22 2014 - 03:37:40 CET