Re: Expressions versus the value they represent

On Apr 12, 1:13 am, David BL <davi..._at_iinet.net.au> wrote:
> If
> this is how the RM is meant to fit into the picture

You have a lot of confusions and misunderstandings about FOPL, relations, RVAs, Prolog and programming abstraction.

If you would properly write out the simplest database some of this would begin to become apparent to you.

What are the names of your relation variables and constants? What is the characteristic predicate
  (parameterized statement about the world) for each? What is the formal wff for each? Why?
What are the attributes for each? Why?
What is your (example) query relation expression? What is the characteristic predicate of its result? Why? What are the (example) values of your relation variables? What statement does each make about the world? Why? What statement does the database make about the world? Why? What is the value of your query relation expression? Why? What statement does this value make about the world   in the context of this query relation expression? Why? What statement does a query result make about the world   regardless of its query relation expression? Why?

The most important things to understand are that: 1. each relation expression corresponds to a certain wff and 2. equivalent relation expressions correspond to equivalent wffs The relational algebra is just another syntax for FOPL. (So they can't possibly be at odds.)

The meaning of an RVA or a TVA, as with any attribute, is whatever the predicate of its containing relation gives it. Any attribute value could denote an abstract value or just itself. The only thing special about an RVA is that it is relation-valued. Usually it denotes a set of tuples that satisfy a predicate (the usual role of a relation value in database management). The only thing special about a TVA is that it is tuple-valued. Usually it denotes either a tuple that satisfies a predicate (the usual role of a tuple value in database management) or it denotes a publicly complex/structured value. Other idioms complicate querying.

philip Received on Wed Apr 14 2010 - 23:01:50 CEST