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_at_pd7urf3no>
...
>
> Hi Jan,
>
> That constraint looks like a tautology to me. Can you explain how any
> relation with a B attribute could fail the constraint?
Date: Fri, 15 Jun 2007 15:08:47 GMT
Message-ID: <3Yxci.30514$1i1.5118_at_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:
R1:
A B
1 1
1 2
2 1
gB
{1,2}
R2:
gB
{1,2}
{1}
p Received on Fri Jun 15 2007 - 17:08:47 CEST