Re: NULLs: theoretical problems?
From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 10 Aug 2007 02:34:44 GMT
Message-ID: <89Qui.45623$fJ5.26414_at_pd7urf1no>
>
>
> Good catch. It seems that logical implication is not well defined for
> three-value logic. According to Lex de Haan and Toon Koppelaars' recent
> book:
>
> P | Q | P->Q
> --------------
> T | U | U
> F | U | T
> U | T | T
> U | F | U
> U | U | U or T?
>
> So possibly I have to modify what I said about no attribute with nulls
> satsifying any FD. Although some dependencies may be satisfied it won't be
> enough for 2NF or anything higher because {K}->{N} is never true where K is
> a key and N is an attribute which has nulls.
>
> The fact that attributes with nulls don't satisfy join dependencies remains
> very solid and much more obvious.
>
Date: Fri, 10 Aug 2007 02:34:44 GMT
Message-ID: <89Qui.45623$fJ5.26414_at_pd7urf1no>
David Portas wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
> news:JXLui.45171$rX4.26997_at_pd7urf2no...
>> (even though I'm not sure in "s{X} = t{X} implies s{Y} = t{Y}" whether
>> "implies" stands for logical implication.)
>>
>
>
> Good catch. It seems that logical implication is not well defined for
> three-value logic. According to Lex de Haan and Toon Koppelaars' recent
> book:
>
> P | Q | P->Q
> --------------
> T | U | U
> F | U | T
> U | T | T
> U | F | U
> U | U | U or T?
>
> So possibly I have to modify what I said about no attribute with nulls
> satsifying any FD. Although some dependencies may be satisfied it won't be
> enough for 2NF or anything higher because {K}->{N} is never true where K is
> a key and N is an attribute which has nulls.
>
> The fact that attributes with nulls don't satisfy join dependencies remains
> very solid and much more obvious.
>
Very good but I must confess that I wasn't thinking that far. I was merely guessing that logical implication with two truth values would mean that if s{X} = t{X} were false, s{Y} = t{Y} would be true, which can't always be the case. Maybe I've got logical implication misconstrued, but I might've got as far as you if Alice had said something like "whenever s{X} = t{X}, s{Y} = t{Y}".
p Received on Fri Aug 10 2007 - 04:34:44 CEST