Re: What to call this operator?

Jon Heggland wrote:
>
> I quite liked the symmetry of the join and union of Tropashko's lattice
> algebra---that the attributes of a join result is the union of the
> attributes of its operands, and the attributes of a union result is the
> intersection of the attributes of its operands.
>
> However, the symmetry/duality is lost in the semantics of the
> operations---the relation predicate of the result. If relvars A and B
> have predicates PA and PB, respectively, the expression A JOIN/<AND> B
> has the predicate PA AND PB (logical and) in both D&D and Tropashko.
> However, D&D's A <OR> B has the predicate PA OR PB (logical or), while
> Tropashko's A UNION B has a rather more complicated and less intuitive
> predicate (on first glance).

I look at it this way:

(I'll call Tropashko's operator "generalized union.")

A natural join B: the union of the columns and the intersection of the rows

A generalized union B: the intersection of the columns and union of the rows.

Quite symmetric, as I see it.

Marshall Received on Wed Jun 29 2005 - 19:36:41 CEST