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Home -> Community -> Usenet -> comp.databases.theory -> Re: TRUE and FALSE values in the relational lattice
On 21 jun, 20:39, "David Cressey" <cresse..._at_verizon.net> wrote:
> "Vadim Tropashko" <vadimtro_inva..._at_yahoo.com> wrote in message
>
> news:1182446779.883211.130730_at_e9g2000prf.googlegroups.com...
>
>
>
> > On Jun 21, 3:14 am, Jan Hidders <hidd..._at_gmail.com> wrote:
> > > I would assume the following operations: (in yet another notation)
> > > - E * E : natural join
> > > - E + E : generalized union
>
> > Eh, 3 different people, 3 different notations:-)
>
> > > - {()} : the relation with empty header and a single empty tuple
>
> > OK
>
> > > - [A,B,..D] : the empty relation with header {A, B, ..., D} (possibly
> > > the empty set)
>
> > This is nice proposal. How abotut small letters for attributes and
> > capital letters for relations?
>
> Why not the following?
>
> {A,B,..D ()}
Because that seems to imply it contains the empty tuple, but it contains no tuples. Also if A, B, .., D is empty this becomes {()} but that is already something else, and yes, I do need a symbol for [] (i.e., the empty relation with the empty header).