Re: A Simple Notation

From: Jan Hidders <>
Date: Tue, 10 Jul 2007 15:13:31 -0000
Message-ID: <>

On 10 jul, 16:38, "David Cressey" <> wrote:
> "Brian Selzer" <> wrote in message
> news:hGLki.45816$
> > By the definition of a natural join, there can only be one relationship.
> In
> > order for a tuple to appear in the result, there must be value agreement
> for
> > *all* common attributes in the tuples from the operands.
> Agreed. But the fact that common attributes are discovered via common names
> is surely a setback (IMO).

Indeed, it is well known and provable that the class of queries expressible with {select, project, join, union, difference} is a proper subset of those expressible in the algebra that also includes the renaming operation, even if your query is a yes/no query, i.e. returns a relation with no columns.

  • Jan Hidders
Received on Tue Jul 10 2007 - 17:13:31 CEST

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