Re: cdt glossary 0.1.1 (repost)

"mAsterdam" <mAsterdam_at_vrijdag.org> wrote:

> [NULL]
> Roughly: a special marker that can be put in a place
> inside a data structure where an actual value is expected.
> Precisely 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 a 'name' 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').
>
>
> Common usage:
>
> - Confusion arises when people use terms like "null value",
> a paradox to some, a contradictio in terminis to others.
>
> - Confusion arises due to the fact that nullness (the absence
> of value) is often represented on computers by the number 0.
> (Obviously, 0 is not null.)
>
> - In some contexts, 'null' and 'nil' mean the same thing;
> in others, they do not.

Everything looks good to this point.
But I cant understand the meaning of the following sentence ....

> In databases traditionally NULL is used and and opposed. ***
> If you want to go into this, please first search for
> mu NIL void NULL undef, 2VL 3VL.

What does the first sentence mean?

BTW, this FAQ has been greatly expanded since the last time I read it, and represents a valuable resource --- ONYA. Well done. Received on Sat Feb 10 2007 - 06:28:36 CET