Re: object algebra
Date: 24 Feb 2004 13:08:37 -0800
Message-ID: <4b45d3ad.0402241308.680a4901_at_posting.google.com>
> In any event, a relation is a predicate.
The above definition of a relation is significantly incomplete according to RDM. See Date's Chapter 4, section 3.
> In short, you've opted out of saying something specific,
> and want to hand-wave about the other attributes.
Sometimes it is reality doing the hand-waving. For example a person may not have any eyes, but the relation's heading has eyeColor defined.
> Which is fine, given adequate subtyping -
> you can model a predicate over a type, and have subtype values
> in attributes of tuples in the relation.
The core problem is not about subtyping but RDM's failure to handling missing data as describe by Date in Chapter 20, Missing Information. Received on Tue Feb 24 2004 - 22:08:37 CET