Re: Examples of SQL anomalies?
Date: Sat, 05 Jul 2008 12:55:20 GMT
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. Received on Sat Jul 05 2008 - 07:55:20 CDT