Re: NULL

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Wed, 08 Jun 2005 22:48:18 +0200
Message-ID: <42a7598d$0$54224$e4fe514c@news.xs4all.nl>

Jan Hidders wrote:
> 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.

s/age/name/ , right?

> (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').

Thank you, Hidders-sensei :-) Received on Wed Jun 08 2005 - 15:48:18 CDT