Re: Types and "join compatibility"

"André Ncss" <andre.naess_at_gmail.com> wrote in message news:1123181919.455069.151790_at_g49g2000cwa.googlegroups.com...

>But as I thought about this I realized that it would be much simpler
>to consider the basic operators. We should really just have to
>consider union, difference, restrict, project and product. Of these,
>the only one that requires us to think about types is clearly
>union.

And difference and restrict.

>One thing that troubles me is the
>relationship between types and constraints.
>To me it seems that types
>and constraints overlap to a large degree.

One thing that troubles me is the
relationship between types and relationships.

A mathematical relation is a subset of a cartesian product of some sets. As a set it can be defined by :
a) enumeration
b) properties

A database relation can be defined by:

a) the system
b) enumeration
c) relational expression

A TTM scalar type can be defined by

a) the system
b) union of some types - union type
c) cartesian product of some types
d) restriction on a cartesian product of some types

The case d) define a subset of a cartesian product of some sets which by definition is a relation (not a relvar).

What is the active domain in the d) case ? Received on Mon Aug 08 2005 - 09:03:32 CEST