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

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

The closed world assumption doesn't mean that unknown values are false. What it means is that when there isn't a row in a table, the fact represented is false. A row with a null is still a row, so the fact represented is true. Received on Fri Aug 14 2009 - 17:16:23 CEST