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

On Thursday, December 18, 2014 2:11:19 AM UTC-8, ruben safir wrote:

> It makes little sense as it is presented,

It makes sense.

> Why do we make them Free or Bound?

You do not need to know why it matters in order to memorize a definition.

It seems like you think it matters. If you stop trying to absorb what the text is saying once you start wondering "why" then you are needlessly stopping yourself from learning.

You may not be used to the size of chunk of definitions necessary for explaining complex things precisely.

Here is a technique which Richard Feynman recommended to his future-astronomer sister when he gave her a college text on astronomy because of her interest in that topic when she was in high school (a technique which I, a person in a very high percentile of reading comprehension, used and recommended before I learned about that): start at the beginning; read as far as you can; repeat. That's just how things are.

> QUOTE
> Informally, a tuple variable t is bound if it is quantified, meaning that it appears in an (∃t) or (∀t) clause; otherwise, it is free.
> ENDQUOTE
>
> ??????

Another definition. If you think that's poor writing, you have some misconceptions about learning advanced material.

> Stop this author doesn't know the material. Either that or he doesn't
> know how to explain it. You can't LEARN math by memorizing rules inside
> rule inside rule.

You are mistaken.

The topic is about a kind of expression. The expressions have a recursive structure. For clarity, the definitions have a similar recursive structure. Therefore, some parts of the definition must be given using terms that are not yet defined.

> QUOTE:
> All free occurrences of a tuple variable t in F are bound in a formula
> F' of the form F' = (∃ t)(F) or F' = (∀t)(F).
> UNQUOTE
>
> He just said above that any ∃t or ∀t means that t is BOUND.

That's what they just said: free in F means bound in (Q t)(F).

Anyway, it was said above "informally", so it wouldn't matter. Initial informality is an effective pedagogic technique. If you think that an introductory statement labelled informal contradicts a later statement labelled formal then you are unfamiliar with precise presentations of complex material.

> Now it is free?

You have failed to notice the difference between names F and F'. If you don't notice such distintions then you aren't going to be able to understand precise writing.

> e) Retrieve the names of employees who work on every project:
>
> This question is insane with SQL and Relational Algebra (where we can
> use a division) but solving it with relational calculus??

And yet you were given a definition of relation division in terms of calculus-like algebraic operators but you mocked it in another message.

This is perhaps histrionic and self-serving considering that the textbook authors, their publisher's editors, your and other instructors that have used the book with students through six editions seem to think it reasonable. If you use this as an excuse to not persist in reading then you are stopping yourself from learning.

philip Received on Sun Dec 21 2014 - 10:41:06 CET