Re: Relation subset operators
From: Tegiri Nenashi <TegiriNenashi_at_gmail.com>
Date: Wed, 3 Jun 2009 17:43:49 -0700 (PDT)
Message-ID: <510296c4-2625-4fad-84e3-5c0de97dc722_at_h28g2000yqd.googlegroups.com>
Date: Wed, 3 Jun 2009 17:43:49 -0700 (PDT)
Message-ID: <510296c4-2625-4fad-84e3-5c0de97dc722_at_h28g2000yqd.googlegroups.com>
On Jun 3, 5:14 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> from an
> algebraic viewpoint, I see aggregates as being very similar to TCLOSE,
> they must be applied first, before restriction. Are you following me?.
TClose is an unary operation borrowed from [binary] relation algebra. It applies to a binary relation (with the arguments of the same type), and can't be applied to a relation with any other signature (unless it is projected to a binary relation). Aggregate and set operations are not unary but binary operations, and both operands are required to have an attribute of the same type. In a way this is a dual requirement to having two argument of the same type! Received on Thu Jun 04 2009 - 02:43:49 CEST