Re: NULL
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Thu, 09 Jun 2005 16:58:54 GMT
Message-ID: <iz_pe.115117$EF7.6590492_at_phobos.telenet-ops.be>
>>mAsterdam wrote:
>>
>>>What makes it worse: NULL is a special case.
>>>People tend to just use the term and assign
>>>to it whatever meaning they want - even different
>>>meanings in different posts.
>>>
>>>So, I think a better text is needed.
>>
>>Well, you should at least explain the following:
>>
>>A null value can be roughly defined as a special marker that can be put in
>>a place inside a data structure where an actual value is expected. The
>>precise semantics of what that marker means varies and there are at least
>>three possibilities that are sometimes assumed:
>>
>>(1) "Unknown value" This means that on the place of the marker there
>>should actually be a value but this value is not known at the present
>>time. For example, if an 'age' field in a tuple describing a person is
>>'null' then this person will have a name but we don't know it.
>>
>>(2) "Absent value" This means that the property that is described by the
>>value in question is simply not defined. For example, if the
>>'shipping-date' field in a tuple describing an order is 'null' then the
>>order was not shipped yet.
>>
>>(3) "Whatever SQL says it means" The exact meaning is hard to summarize
>>briefly, but is a mixture of the previous two interpretations and involves
>>a value with three truth-values ('true', 'false' and 'unknown').
Date: Thu, 09 Jun 2005 16:58:54 GMT
Message-ID: <iz_pe.115117$EF7.6590492_at_phobos.telenet-ops.be>
mountain man wrote:
> "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"?
Read closely, that is not precisely how he defines it.
- Jan Hidders