Re: constraints in algebra instead of calculus

On 15 jun, 18:26, paul c <toledobythe..._at_oohay.ac> wrote:
> Jan Hidders wrote:
> > On 15 jun, 16:32, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>Jan Hidders wrote:
>
> ...
>
> >>That constraint looks like a tautology to me. Can you explain how any
> >>relation with a B attribute could fail the constraint?
>
> > I'm not that well versed in the TTM / Tutorial D notation so I may
> > have abused the notation a bit. To clarify:
> > - B is a set of attributes (the non-key attributes)
> > - R{B} denotes the projection of R on the attributes in B
> > - R GROUP {B} AS C groups the attributes in B and names the resulting
> > set-valued attribute C
>
> > That probably clears it up, but I'll give a small example anyway
> > (note: here B is not a set of attributes but a single attribute):
>
> > Assume R = { (A:1, B:2), (A:1, B:3) }
>
> > R{B} = { (B:2}, (B:3) }
> > R1 = R{B} GROUP {B} AS gB = { (gB:{ (B:2} }), (gB:{ (B:3) }) }
> > ...
>
> I take it that R1 here has two tuples?

Yes, the tuples (gB:{ (B:2) }) and (gB:{ (B:3) }). I'm using the usual set enumeration notation, so { t_1, t_2 } denotes a set with two elements, t_1 and t_2.

• Jan Hidders