Re: completeness of the relational lattice

On 22 jun, 19:14, Vadim Tropashko <vadimtro_inva..._at_yahoo.com> wrote:
> On Jun 22, 9:42 am, Jan Hidders <hidd..._at_gmail.com> wrote:
>
>
>
> > Marshall schreef:
> > > Any expression of the form
>
> > > E \/ [x]
>
> > > can be rewritten as
>
> > > E \/ (X /\ 00)
>
> > > where X is any relation with the same header as [x].
>
> > ?? Either you can express it or you cannot. Either you can give me an
> > expression equivalent to [x] or you cannot. An expression that is only
> > equivalent under certain circumstances is not good enough. It means
> > your expressive power in terms of queries that you can express drops
> > below that of the unions of conjunctive queries (UCQ). If the database
> > consists of just R(a,b) then how can I project on {a}, or on {c,d}?
>
> You cannot. [...]

Indeed. So can we agree on my addition for the time being?

• Jan Hidders