Re: Lots of Idiotic Silly Braces?

From: paul c <>
Date: Fri, 20 Jul 2007 15:39:34 GMT
Message-ID: <WG4oi.131848$xq1.57098_at_pd7urf1no>

paul c wrote:
> Brian Selzer wrote:
> ...

>> definition of a functional dependency, it follows that the duality of 
>> GROUP and UNGROUP can only hold when the rva is a dependent attribute.
>> ...

> It holds when the attributes that are not grouped are dependents.

Pardon me, I didn't say that right. Using my own oddball notation which helps me see a little easier how many tuples there are, let me try again.

(1) r=
a b
1 1
1 2
2 2

(2) r{S{a},b}= // (ie., group r, on a) S{a} b
{1} 1
{1,2} 2

(3) S{r{a}}= // (ie., group the projection on a of r, on a S{a}

The projection "S{a}" from (2) and the value of (3) are different because r.a is not a determinant. Whereas if the second tuple of r were removed, they would be equal.

p Received on Fri Jul 20 2007 - 17:39:34 CEST

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