Re: Love or hate, or? domains with cardinality two

Op woensdag 11 november 2015 19:36:37 UTC+1 schreef Nicola:
> On 2015-11-10 18:21:54 +0000, Gene Wirchenko said:
> >
> >>> And what obvious difficulties?
> >>
> >> If you think there aren't any, show me a straightforward solution (of
> >> course, what is difficult and what is not is largely subjective!).
> >
> > I have not claimed that there were none.
> >
> > You are the one who made the claim that they were obvious. Prove
> > your claim.
>
> On the Employee(id, dept, mgr : boolean) schema, the constraint by which
> at any time each department has one and only one manager may be formalized
> as follows:
>
> forall x (Department(x) -> (exists y (Employee(x, y, true)
> and
> forall w,z (Employee(x, w, z) and w <> y -> z = false))))
>
> This is not the most straightforward constraint

???

(DEPARTMENT NOTMATCHING EMPLOYEE) SUBSET-OF FI plus a key declaration {dept} on the view EMPLOYEE WHERE mgr

> Of course, if you reject the constraint as invalid to begin with, this
> whole point is void.
>
> Nicola
>
Received on Mon Nov 16 2015 - 21:34:27 CET