Re: Relational symmetric difference is well defined
From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Sat, 16 Jun 2007 16:16:46 +0200 (CEST)
Message-ID: <Xns995168E7F90F4vdghher_at_194.177.96.26>
>
> Something is not right here. So, join, union, and difference can be
> defined via first order formulas, while relational division don't?
> Sure relational division is expressible via join, union, and
> difference!
Date: Sat, 16 Jun 2007 16:16:46 +0200 (CEST)
Message-ID: <Xns995168E7F90F4vdghher_at_194.177.96.26>
Vadim Tropashko <vadimtro_invalid_at_yahoo.com> wrote in news:1181951119.615693.101990_at_d30g2000prg.googlegroups.com:
> On Jun 15, 4:21 pm, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
>> Sure, second order lang. is more expressive, but its expressivity
does
>> not come for free !
>
> Something is not right here. So, join, union, and difference can be
> defined via first order formulas, while relational division don't?
> Sure relational division is expressible via join, union, and
> difference!
Of course it is, e.g.(project def. project on some attributes A not in R2):
R1 divide R2 = project(R1) - project(project(R1) times R2) - R1).
Converting the above to a first-order formula is left as an exercise;)
>
>
Received on Sat Jun 16 2007 - 16:16:46 CEST