Re: Plural or singular table names

"Bob Badour" <bbadour_at_golden.net> wrote in message news:TpH5b.479$rt5.47174800_at_mantis.golden.net...
> "Marshall Spight" <mspight_at_dnai.com> wrote in message
> news:bqz5b.263499$Oz4.69861_at_rwcrnsc54...
> >
> > The question being discussed is whether relation values have constraints,
> > and in particular whether they have key constraints. I can see how the
> above
> > explanation works if I have already accepted the idea that relation
> > values have keys, but I do not see how it leads me to accept that
> > idea if I haven't already.
>
> Are you suggesting you reject the information principle and logical
> identity?

Of course not, and I have no idea why you even brought it up.

> > You also now seem to be including the idea that constraints
> > are part of the type. This also strikes me as unusual.
>
> Specialization by Constraint seems unusual to you? As soon as you take one
> type and constraint it, you have created a new type.

We were speaking of constraints in general, and key constraints in particular. Now you are saying that type constraints and key constraints are the same thing? That is the only way I can make sense out of your response.

TTM draws the distinction among type constraints, attribute constraints, relvar constraints, and database constraints.

Type constraints are constraints on types, (not relational values.) Attribute constraints are constraints on the legal values of a given   attribute within a relvar.
Relvar constraints are constraints on relvars, not relations. Database constraints are constraints on two or more specific relvars.

Your have been saying that relation values have key constraints. Which of the above kinds of constraints is a key constraint on a relation value? Or do you have a different taxonomy you could tell us about?

> I don't see that. If all relations have logical identity, they all have at
> least one irreducible key. If all relations have one irreducible key, they
> all have logical identity. Values, variables, expressions and literals do
> not come into consideration.

Okay. You're saying that the uniqueness property is the same thing as a key constraint. I can see that, although I'm not sure I agree. The definition of "key" now becomes relevant.

[Checking TTM...]

TTM's definition of candidate key specifies that it applies only to relvars. (See RM Prescription 15.)

Perhaps you have a different definition of key that you could tell us about?

> > Also, I can see how:
> >
> > every relation has a key -> unique tuples
> >
> > but I don't see how
> >
> > unique tuples -> every relation has a key.
>
> The latter implies that the set of all attributes is a (possibly
> irreducible) superkey, which implies that the relation has at least one key.

Again, this depends on the definition of "key."

> > Foldoc gives this definition for constraint: "A Boolean relation,
> > often an equality or ineqality relation, between the values of
> > one or more mathematical variables (often two)."
> >
> > Also, I checked a few places in TTM, but could only find references
> > to constraints on variables (database or relation variables) and
> > constraints on types. (Admittedly, this is just appeal to authority,
> > and not definitive.)
>
> You found no reference to specialization by constraint or to generalization
> by constraint?

Note I said "and constraints on types."

> You found no suggestion that values have types?

I don't see how you get from what I said to that.

Marshall Received on Thu Sep 04 2003 - 17:29:14 CEST