# Re: constraints in algebra instead of calculus

Date: Tue, 19 Jun 2007 12:39:32 GMT

Message-ID: <88Qdi.41068$5j1.38672_at_newssvr21.news.prodigy.net>

"David Cressey" <cressey73_at_verizon.net> wrote in message
news:TLOdi.3043$Sm5.2612_at_trndny04...

*>
*

> "Jan Hidders" <hidders_at_gmail.com> wrote in message

*> news:1182241089.155428.89490_at_q69g2000hsb.googlegroups.com...
**>> On 19 jun, 08:26, "David Cressey" <cresse..._at_verizon.net> wrote:
**>> > "paul c" <toledobythe..._at_oohay.ac> wrote in message
**>> >
**>> > news:vNFdi.37701$NV3.32514_at_pd7urf2no...
**>> >
**>> >
**>> >
**>> >
**>> > I can't see much use for grouping on all attributes. It seems to me
**> that
**>> > this has to be a null operation.
**>>
**>> That would actually make the math harder, which is often a bad sign.
**>> Right now the definition is quite simple:
**>>
**>> R GROUP A AS B = { t[H-A] + (B : { t'[A] | t' in R, t'[H-A]=t[H-A] })
**>> | t in R }
**>>
**>> where
**>> - H is the set of all attributes of R
**>> - t[X] is the projection of tuple t on the set of attributes X
**>> - + is tuple concatenation
**>> - (B : v) constructs a tuple with a single field B with value v
**>>
**>> If you let A be equal to H then
**>> - H-A is the empty set
**>> - t[H-A] is the empty tuple () for all t
**>> - t[A] = t for all t in R,
**>> so you get:
**>>
**>> { () + (B : { t' | t' in R, ()=() }) | t in R } =
**>> { (B : { t' | t' in R }) | t in R } =
**>> { (B : { t' | t' in R }) } =
**>> { (B : R) }
**>>
**>> Your suggestion would create an exception to that rule.
**>
**> It wasn't my intent to make a suggestion. It was supposed to be an
**> observation. An incorrect one, it would appear from your response.
**>
**> It sounds like I'm mixed up on what GROUP does. In particular, it sounds
**> like I was confusing
**>
**> R GROUP A AS B with
**> R GROUP (H-A) AS B
**>
**> Where can I go for a definition of GROUP?
**>
*

paul c provided a link to Appendix A of TTM earlier. It is also described
in Date's /An Introduction to Database Systems, Eighth Edition/.
Received on Tue Jun 19 2007 - 14:39:32 CEST