Re: Examples of SQL anomalies?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 05 Jul 2008 10:49:41 -0300
Message-ID: <486f7bfa$0$4047$9a566e8b_at_news.aliant.net>
>
> If the schema specifies that the weight attribute is nullable,
> we need to know what, precisely, is the intended meaning of a
> null weight. If may mean that the weight is unknown, or it may
> mean that the weight attribute is not applicable to this entity
> (e.g. the weight of an electronic book), or it may mean ...
> In a badly designed schema it may unfortunately be used with
> more than one meaning without permitting them to be distinguished.
>
> Nulls are often but not exclusively used to mean unknown value.
> Failure to distinguish between different uses of null is the
> basis of most of these, rather pointless, arguments.
Date: Sat, 05 Jul 2008 10:49:41 -0300
Message-ID: <486f7bfa$0$4047$9a566e8b_at_news.aliant.net>
goanna wrote:
> Marshall <marshall.spight_at_gmail.com> writes:
>
>>What can be meaningfully asked is determined by the schema.
>
> Close. What can be meaningfully asked is determined by the meaning
> of the schema. In the case of nullable attributes, this must be
> specified, not guessed.
>
>
>>If the schema specifies that the weight attribute is nullable, >>then the question of how much a shipment weighs in total >>is a question that cannot be asked.
>
> If the schema specifies that the weight attribute is nullable,
> we need to know what, precisely, is the intended meaning of a
> null weight. If may mean that the weight is unknown, or it may
> mean that the weight attribute is not applicable to this entity
> (e.g. the weight of an electronic book), or it may mean ...
> In a badly designed schema it may unfortunately be used with
> more than one meaning without permitting them to be distinguished.
>
> Nulls are often but not exclusively used to mean unknown value.
> Failure to distinguish between different uses of null is the
> basis of most of these, rather pointless, arguments.
I disagree. Ignorance is the basis of these rather pointless arguments. The whole idea of NULL or missing markers has been thoroughly refuted as a useful idea. Received on Sat Jul 05 2008 - 15:49:41 CEST