Re: thinking about UPDATE

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:pan.2004.07.24.09.17.01.852618_at_REMOVETHIS.pandora.be...
> On Sat, 24 Jul 2004 02:50:53 +0000, Marshall Spight wrote:
> >
> > Ah, I misunderstood. Although actually, I can think of a use for a
> > column with only one possible value in a relation where there is more
> > than one tuple.
> >
> > Let's say you have a relation Base, with one of two possible "subtype"
> > relations T1 and T2. You wish to enforce that each row in the base table
> > has a corresponding row in either T1 or T2, but not both. Declare an
> > enumerated type TT { t1, t2 }. Declare subtypes TT1 = { t1 } and TT2 = {
> > t2 }.
> > Declare the key of Base to be { id:int, t:TT }. Declare the key of T1 to
> > be { id:int, t:TT1 } references Base(id,t) Declare the key of T2 to be {
> > id:int, t:TT2 } references Base(id,t)
> >
> > There is no type error in joining, because TT1 <: TT and TT2 <: TT.
>
> You still haven't modelled that for all tuples in Base there should be a
> corresponding tuple in either T1 or T2.

D'oh!

> I agree that this is a nice trick,
> but it is a trick, and you wouldn't need it if the constraint language of
> the database was powerful enough.

I don't know; I think these kinds of structural ways of doing things are better even than declarative constraints.

> > PS. Seriously, what does JD stand for?
>
> Join dependencies, the most general of them all and used to define 5NF.
> They consist of a set of headers and say that the corresponding
> decomposition is information preserving. So for a relation R(a,b,c) the
>
> JD({a,b}, {a,c}, {b,c})
>
> says that it always holds that
>
> R[a,b] [X] R[a,c] [X] R[b,c] = R
>
> where R[a,b] denotes the projection of R on {a,b} and [X] is the natural
> join.
>
> The usual example is DrinkAt(drinker, bar, beer) with the constraint that
> if (1) a drinker drinks a certain beer, (2) a bar servers that beer and
> (3) that drinker visits that bar then it follows that this drinker drinks
> that beer in that bar.

Cool.

Marshall Received on Sat Jul 24 2004 - 16:54:33 CEST