Re: NULL

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:ekIpe.114528$Po5.6677642_at_phobos.telenet-ops.be...
> 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"?

"Null values (distinct from the empty character string or a string of blank characters and distinct from zero or any other number) are supported in fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of data type."

If data does not exist or does not apply then a value of NULL is applied, this is understood by the RDBMS as meaning non-applicable data."

-- 
Pete Brown
Falls Creek
OZ
www.mountainman.com.au