Re: NULLs: theoretical problems?
From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Sat, 11 Aug 2007 02:33:34 +0200 (CEST)
Message-ID: <Xns9988D125DB68vdghher_at_194.177.96.26>
>> "paul c" <toledobythe..._at_oohay.ac> wrote in message
>>
>> news:vnIui.42734$_d2.2251_at_pd7urf3no...
>>
>>
>>
>> > Hugo Kornelis wrote:
>> >> On Wed, 08 Aug 2007 20:42:52 GMT, paul c wrote:
>>
>> >>> Hugo Kornelis wrote:
>> >>>> ...
>> >>>> The ANSI standard defines NULL as follows:
>>
>> >>>> "null value: A special value that is used to indicate the
>> >>>> absence of any data value."
>>
>> >>>> Thus, the only concept collapsed into NULL is that of "no value
>> >>>> here". ...
>> > ...
>> >> <nitpicking>
>> >> The ANSI text doesn't define "the null value" (as they call it) as
>> >> a value that conveys that the value is not _a_ value, but as a
>> >> _special_ value that conveys that there is no _data_ value.
>> >> </nitpicking>
>>
>> >> Personally, I never call NULL a value, and I prefer to describe
>> >> NULL as a "marker that indicates the absence of any value". But I
>> >> couldn't really write that and still claim to be citing ANSI, eh?
>> >> ...
>>
>> > I noticed in your blog you said that the table with a null-able
>> > birthday column was in 2NF. If saying that is right, I suppose we
>> > must be careful to regard functional dependencies as determining
>> > values sometimes and non-values other times.
>>
>> But Hugo is not right. From the Alice Book, p163:
>>
>> "A relation I over U satisfies X -> Y, if for each pair s, t of
>> tuples in I, s{X} = t{X} implies s{Y} = t{Y}."
>>
>> Since null = null is not true, no attribute with nulls satisfies any
>> FD, not even the trivial one {A}->{A}!
Date: Sat, 11 Aug 2007 02:33:34 +0200 (CEST)
Message-ID: <Xns9988D125DB68vdghher_at_194.177.96.26>
Jan Hidders <hidders_at_gmail.com> wrote in news:1186742457.327000.136820_at_x40g2000prg.googlegroups.com:
> On 9 aug, 23:25, "David Portas" > <REMOVE_BEFORE_REPLYING_dpor..._at_acm.org> wrote:
>> "paul c" <toledobythe..._at_oohay.ac> wrote in message
>>
>> news:vnIui.42734$_d2.2251_at_pd7urf3no...
>>
>>
>>
>> > Hugo Kornelis wrote:
>> >> On Wed, 08 Aug 2007 20:42:52 GMT, paul c wrote:
>>
>> >>> Hugo Kornelis wrote:
>> >>>> ...
>> >>>> The ANSI standard defines NULL as follows:
>>
>> >>>> "null value: A special value that is used to indicate the
>> >>>> absence of any data value."
>>
>> >>>> Thus, the only concept collapsed into NULL is that of "no value
>> >>>> here". ...
>> > ...
>> >> <nitpicking>
>> >> The ANSI text doesn't define "the null value" (as they call it) as
>> >> a value that conveys that the value is not _a_ value, but as a
>> >> _special_ value that conveys that there is no _data_ value.
>> >> </nitpicking>
>>
>> >> Personally, I never call NULL a value, and I prefer to describe
>> >> NULL as a "marker that indicates the absence of any value". But I
>> >> couldn't really write that and still claim to be citing ANSI, eh?
>> >> ...
>>
>> > I noticed in your blog you said that the table with a null-able
>> > birthday column was in 2NF. If saying that is right, I suppose we
>> > must be careful to regard functional dependencies as determining
>> > values sometimes and non-values other times.
>>
>> But Hugo is not right. From the Alice Book, p163:
>>
>> "A relation I over U satisfies X -> Y, if for each pair s, t of
>> tuples in I, s{X} = t{X} implies s{Y} = t{Y}."
>>
>> Since null = null is not true, no attribute with nulls satisfies any
>> FD, not even the trivial one {A}->{A}!
> > A small warning. You are taking a predicate over a certain domain > (namely relations as defined in the Alice book) and are now trying to > apply to things outside that domain (relations with null values) for > which it not necessarily makes sense. Of course, it wil still lead to > interesting conversation and lots of armchair philosophizing, but > probably will not produce anything meaningful.
Right on spot. The 'Alice' book has nothing to say about nulls.
> > -- Jan Hidders > >Received on Sat Aug 11 2007 - 02:33:34 CEST