From: Jan Hidders <>
Date: Thu, 09 Jun 2005 16:58:54 GMT
Message-ID: <iz_pe.115117$>

mountain man wrote:

> "Jan Hidders" <> wrote in message 
> news:ekIpe.114528$

>>mAsterdam wrote:
>>>What makes it worse: NULL is a special case.
>>>People tend to just use the term and assign
>>>to it whatever meaning they want - even different
>>>meanings in different posts.
>>>So, I think a better text is needed.
>>Well, you should at least explain the following:
>>A null value can be roughly defined as a special marker that can be put in
>>a place inside a data structure where an actual value is expected. The
>>precise semantics of what that marker means varies and there are at least
>>three possibilities that are sometimes assumed:
>>(1) "Unknown value" This means that on the place of the marker there
>>should actually be a value but this value is not known at the present
>>time. For example, if an 'age' field in a tuple describing a person is
>>'null' then this person will have a name but we don't know it.
>>(2) "Absent value" This means that the property that is described by the
>>value in question is simply not defined. For example, if the
>>'shipping-date' field in a tuple describing an order is 'null' then the
>>order was not shipped yet.
>>(3) "Whatever SQL says it means" The exact meaning is hard to summarize
>>briefly, but is a mixture of the previous two interpretations and involves
>>a value with three truth-values ('true', 'false' and 'unknown').
> What is lacking Codd's concise definition (rule 3)
> for null data as "non-applicable data"?

Read closely, that is not precisely how he defines it.

  • Jan Hidders
Received on Thu Jun 09 2005 - 18:58:54 CEST

Original text of this message