Re: NULL
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Thu, 09 Jun 2005 06:45:15 GMT
Message-ID: <%zRpe.114763$w26.6794308_at_phobos.telenet-ops.be>
>> mAsterdam wrote:
>>
>>> What makes it worse: NULL is a special case.
>>> People tend to just use the term and assign
>>> to it whatever meaning they want - even different
>>> meanings in different posts.
>>>
>>> So, I think a better text is needed.
>>
>>
>> Well, you should at least explain the following:
>>
>> A null value can be roughly defined as a special marker that can be
>> put in a place inside a data structure where an actual value is
>> expected. The precise semantics of what that marker means varies and
>> there are at least three possibilities that are sometimes assumed:
>>
>> (1) "Unknown value" This means that on the place of the marker there
>> should actually be a value but this value is not known at the present
>> time. For example, if an 'age' field in a tuple describing a person is
>> 'null' then this person will have a name but we don't know it.
>
> s/age/name/ , right?
Date: Thu, 09 Jun 2005 06:45:15 GMT
Message-ID: <%zRpe.114763$w26.6794308_at_phobos.telenet-ops.be>
mAsterdam wrote:
> 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?
Yes. Sorry.
>> (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').
"involves a value" -> "logic"
> Thank you, Hidders-sensei :-)
Thank you, but you will always the the master. :-)
- Jan Hidders