Re: constraints in algebra instead of calculus

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 15 Jun 2007 15:08:47 GMT
Message-ID: <3Yxci.30514$1i1.5118@pd7urf3no>

Bob Badour wrote:
> Jan Hidders wrote:
> ...

>>
>> You can simplify that a bit:
>>
>> R1 := (R{B}) GROUP {B} AS gB
>> R2 := (R GROUP {B} AS gB){gB}
>> constraint: R1=R2

...
>
> Hi Jan,
>
> That constraint looks like a tautology to me. Can you explain how any
> relation with a B attribute could fail the constraint?

Butting in, pardon the sloppy notation, isn't this an example?

R:
A B
1 1
1 2
2 1

R1:
gB
{1,2}

R2:
gB
{1,2}
{1}

p Received on Fri Jun 15 2007 - 10:08:47 CDT