Re: NULL

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').

• Jan Hidders