Date: Tue, 15 Jan 2008 12:48:36 -0800 (PST)
On Jan 15, 11:25 am, Kira Yamato <kira..._at_earthlink.net> wrote:
> On 2008-01-15 13:22:50 -0500, Bob Badour <bbad..._at_pei.sympatico.ca> said:
> > Kira Yamato wrote:
> >> On 2008-01-15 09:26:01 -0500, mAsterdam <mAster..._at_vrijdag.org> said:
> >> This seems right. A domain is just a set of values. In relational
> >> algebra, this set is required to be non-empty since attributes are
> >> non-null.
> > Theoretically, the universal subtype has an empty set of values and the
> > union of all operations.
> Can you explain a bit more what you mean here?
> What is the universal subtype?
In type systems with subtyping, a universal subtype is the type that is a subtype of all types. In lattice terms, it is "bottom".
> If it is an empty set, where is it used?
> Can this type be an attribute in a relation? I would think not, since
> relational algebra requires all values in the tuples be non-null.
A type that includes at attribute of type bottom may exist, but it cannot be instantiated. In relational terms, a relation whose type includes an attribute of type bottom is necessarily an empty relation.
Marshall Received on Tue Jan 15 2008 - 21:48:36 CET