Re: So what's null then if it's not nothing?

From: Alexandr Savinov <spam_at_conceptoriented.com>
Date: Fri, 18 Nov 2005 10:16:07 +0100
Message-ID: <437d9be8$1@news.fhg.de>

David Portas schrieb:
> Alexandr Savinov wrote:
>

>>>Not at all the same thing because nulls violate the Information
>>>Principle, which is a foundation of RM. Nulls as formulated in SQL also
>>>create logical problems that are very hard to solve - at least SQL
>>>hasn't succeeded in solving them very satisfactorily.
>>
>>Maybe, so what do you propose?

>
>
> In place of SQL? The Relational Model would be nice to have one day...
> :-)
>
>

>>>RM does not prohibit such questions. If it is more natural to give some
>>>answer then the user can demand such an answer. The user can decide
>>>what he wants to see in the case of a dog with colour or a dog without
>>>colour. I would challenge your implication that users find nulls"more
>>>natural". I bet most DBMS users wouldn't do a very good job of
>>>explaining what a null is. In fact in many database projects the
>>>developers go to a lot of trouble to hide nulls from regular users.
>>
>>User and developers are not part of a data model.
>>

>
>
> Neither are the questions. Your point was that certain questions were
> forbidden in the absence of nulls. Mine was that users decide on the
> questions and that null isn't an answer that they want or need.
>
>

>>and the same because effectively it will not be visible. Why do we need
>>to mark things/facts as absent by means of nulls if we can simply delete
>>them physically? Because things exist in multidimensional hierarchical
>>space so they exhibit themselves in a more complex manner than
>>exist/non-exist. And of course it is differs from what RM teaches.
>>

>
>
> Can you give an example of how nulls could exhibit information
> information that is somehow "more complex" than can be represented
> without them? This is somewhat more important than your earlier
> assertions that nulls are merely "convenient". If you think nulls are
> _necessary_ to represent certain types of information then you ought to
> be able to demonstrate that by example. It seems plain to me that no
> such example can exist (unless you contrive some data model that is
> very stunted in the types of information it can represent - certainly
> not RM which represents multidimensional information very well).

Here is an example. Our problem domain consists of a set of objects which can be characterized by a set of attributes. This set of objects is rather generic and the expert we are interviewing says that there well may be objects for which any attribute does not make sense.

Then we want to produce correct reports with aggregated information. If we use nulls for inappropriate attributes then the database will (if implemented so) produce correct reports.

Do you have an alternative to this design without nulls?

If you think that this example is too rare then you are right. But is actually not an exception is where many attributes may be absent.

> Like JOG I thought the debate was over. We tolerate nulls, we don't
> like them.

Ok, since the depate is over I will just summarize the positions:

"We tolerate nulls, we don't like them." That is, nulls are necessary evil. We should avoid using them becasue it is the source of many problems. (Reformulate me if I am not correct, because it is not my position.)
Null is one of the most important initial notions and many other notions can be derived from it. The mechanisms and constructs for manipulating and maintaining nulls are of crucial importance. Although we can avoid using nulls this will be a significant limitation both from theoretical and practical points of views.

-- 
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Received on Fri Nov 18 2005 - 03:16:07 CST