Re: WWW/Internet 2009: 2nd CFP until 21 September

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 14 Aug 2009 14:48:24 GMT
Message-ID: <YOehm.41212$PH1.10421_at_edtnps82>



Mr. Scott wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
...
>> 4) if the answer to 2) is 'one predicate', what does projection on any of 
>> the four columns mean?

>
> That is an interesting question. I think that the predicate of a projection
> must include in some way the predicate of its operand. A projection over B
> on the table {X,A,B,C,D,E} includes only those B's that are actually in the
> table. Does this look right to you? "Being a snail-mail address B for an
> order where an order has order number X for customer E that was placed on
> date D with delivery e-mail address A ....

If some 'row' (pretending for the moment that rows can be treated like tuples) contains 'null' (eg., 'unknown') in the B column, I presume in SQL that row instantiates, ie. stands for a true fact. But the Closed World Assumption means that unknown values are false. Seems like a contradiction to me. Received on Fri Aug 14 2009 - 09:48:24 CDT

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