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Re: A Simple Notation

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 06 Jul 2007 23:52:15 GMT
Message-ID: <PAAji.92566$xq1.59731_at_pd7urf1no> 






Brian Selzer wrote:


> "David Cressey" <cressey73_at_verizon.net> wrote in message 


> news:eirji.2$475.1_at_trndny04...

> 

>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>news:UHlji.18386$2v1.9600_at_newssvr14.news.prodigy.net...
>>
>>>"paul c" <toledobythesea_at_oohay.ac> wrote in message
>>>news:4Ifji.90354$xq1.46042_at_pd7urf1no...
>>>




> ... 

> The extension of the predicate of A contains only those tuples for which 


> P(t) is true.  The extension of X MINUS A contains only those tuples for 

> which P(t) is false.  Of course if the domains referenced in A are finite, 

> then the set of n-tuples of objects exemplified by the predicate of A is 

> also finite, so the complement of A is identical to the extension of X MINUS 

> A.




I wonder when, in the course of the usual human affairs as they involve 
the basic relational dbms op's, does it make sense to think of infinite 
domains?  Isn't it usually sufficient, as far as algebra is concerned, 
to pretend they are finite?



Also, while I think complements are important, eg., to understand view 
updating, they do drive me crazy when constraints are present, sometimes 
wondering if they need to be expressed with rva's.  (I'll probably be 
corrected for making that comment, but that's okay, being slow and lazy, 
I enjoy being corrected!)



p
Received on Sat Jul 07 2007 - 01:52:15 CEST












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