Re: Foreign superkey support

Marshall wrote:
> erk wrote:
> >
> > Some database constraints restrict only the domain of tuples allowed in
> > a specific relvar, so are relation constraints (i.e. they are
> > expressions over only the attributes of a single relvar). Other more
> > specific constraints are type constraints and attribute constraints
> > (expressions over a single attribute or attribute type).
>
> I have read various classification systems for constraints, and they
> all seem unnecessary to me. They don't really buy me much if
> anything.

Just to clarify: I was really just playing some mental games with typing, and "how far up it goes": types of attributes, types of tuples, types of relations, types of databases. With respect to that, the "scope" of a constraint does determine whether it's part of the type of a relation or a database.

For example, let's say you have a database D1 with relations A1 {x:X, y:Y} and B1 {x:X, w:W}, with no constraints (the x attribute in A1 and B1 just coincidentally the same, by the way). That database (a variable) has a type/domain, defined by the set of relvars it contains.

If I define D2 like D1 but with a constraint ("all B2.x in A2.x"), then the type/domain of D2 is a subtype of D1. It would also seem that the type of B2 is a subtype of B1, despite the fact that the constraint is a database constraint. I'm not sure about that, and am somewhat worried that I'm stumbling into a variant of one of the Great Blunders here.

> While on the one hand it is quite tempting to consider
> a constraint as having some particular scope, especially if it can
> be pinned on a particular relvar, it doesn't really hold up. For any
> constraint, we have a set of relations that it is quantified over.
> If that set is cardinality one, that is just particular to that
> constraint; it doesn't classify it per se.

But a cardinality 1 constraint definitely defines a relation subtype, whereas cardinality > 1 doesn't (necessarily).

> We don't consider the
> cardinality of one side of other many-to-many relationships as being
> classifying, do we?

I'm not sure.

> In other words, there is a many-to-many relationship between
> constraints and relvars. (Or relations, if we are thinking in
> descriptive rather than prescriptive terms.)

It depends. To what extent are the cardinality-1 constraints part of the "type" of the relvar? That's the only notion I'm playing with.

> For example,
> we often speak of a foreign key constraint as belonging to
> one particular table, but it is equally a constraint on two tables.
> On one table it restricts inserts and on another table it restricts
> deletes.

Good point - so in the above case, is A2 a subtype of A1 and B2 a subtype of B1?

> So we must necessarily classify it as a database
> constraint. But really, all constraints are all database constraints,
> although there may be some database constraints that only
> happen to be quantified over a single relvar.

Yep.

• erk