Re: On specialization constraints time of application
Date: Sun, 14 Jun 2009 05:23:08 -0400
"Walter Mitty" <wamitty_at_verizon.net> wrote in message
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> It is my opinion that nulls are not inherently flawed, but that it is the
>> misinterpretation, and the consequent misuse of nulls that are.
> Having said that, my preferred interpretation of NULL is different from
There are a number of interpretations. Raymond Reiter in "A Sound and Sometimes Complete Query Evaluation Algorithm for Relational Databases with Null Values," Journal of the ACM, April 1986, argues that nulls are constants for which there is no unique name axiom. Codd, in his book, RMv2, states that nulls are not values, but indicators and distinguishes between inapplicable nulls and applicable nulls by using two separate null indicators and 4VL. Others argue that there are many different kinds of null, but what that boils down to is that that there are many different reasons that a value either should have been supplied but hasn't, or is not applicable, and that reason should somehow be encoded as the kind of null value. The interpretation that I take issue with the most is the one that uses the same symbol to indicate both that there can't be a value and that there can be a value but it hasn't been supplied. Received on Sun Jun 14 2009 - 11:23:08 CEST