Re: A Simple Notation

From: Brian Selzer <brian_at_selzer-software.com>
Date: Sat, 07 Jul 2007 03:50:14 GMT
Message-ID: <W3Eji.18503$2v1.12953@newssvr14.news.prodigy.net>

"paul c" <toledobythesea_at_oohay.ac> wrote in message news: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?

>

Why limit yourself to what can be represented in a computer? An algebra that supports relations with infinite cardinality or degree can certainly support any relation that can be represented in a computer.

> 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 Fri Jul 06 2007 - 22:50:14 CDT