Re: header part of the value?

From: Tegiri Nenashi <TegiriNenashi_at_gmail.com>
Date: Wed, 27 Feb 2008 19:32:20 -0800 (PST)
Message-ID: <2baa7327-e108-4dab-a724-3a20de696987_at_41g2000hsc.googlegroups.com>


On Feb 27, 7:08 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
> It appears to me as if you are viewing the
> equation "x + 3 = y" as having 3 as the free variable, and
> x and y as constants! Whereas I would consider it much
> more natural to consider x and y as free variables, or
> parameters, or attribute names (they are all the same)
> and 3 and 5 as constants.

Well, the case with two variables in the header is tricky. First, define union and join then we can talk about it:-)

One more odd thing about the example is that an interesting mathematical case is a system of equations which is conjunction, and not disjunction!

> If I may select and project from your example, consider:
>
>   x>=
>   ---
>   1
>   2
>   5

This is invalid relation. Certainly, you have meant

x>=

---
1


> That to me seems to be simply a special case, that of the natural join
> of the following two relations (specified via comprehension)
>
>   {(x, y) | x >= y}
> and
>   {(y) | y in {1, 2, 5}}
Now that relation collapsed to a single tuple can you please rework the example ? For example, you can add one more attribute x>= z<= --- --- 1 3 2 4 5 8 so that your original relation is a projection of that one. Once again I find it fascinating that the min aggregate of the column x in the relation x z --- --- 1 3 2 4 5 8 (which is equal to 1) is just a projection of generalized relation x>= z<= --- --- 1 3 2 4 5 8 onto the header "x>=". (The exact signature of the column z doesn't matter).
Received on Thu Feb 28 2008 - 04:32:20 CET

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