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

David Portas schrieb:

> Alexandr Savinov wrote:
> 

> 
> 
> This is the heart of the problem you have in mind I think: That a model
> without nulls is more expensive to implement than one with. In many
> current database products that is true, but it isn't a fundamental
> objection. It says more about those database products than about the
> theory.
> 
> 

> 
> 
> If your reality changes then your logical model of it does too. This is
> entirely to be expected. Nulls don't change the picture.

One point is that it is always to predict especially about future. So that any model allows for some degree of flexibility. We cannot hard code into its structure all possible situations.

But even if it would be possible there is another (fundamental) problem. We need nulls. It is a very important mechanism of data modeling. We need to somehow designate the fact of absence of an object. So nulls cannot be avoided -- they can be only ignored, hidden or discredited. Because NULL IS A FACT OF ABSENCE OR NON-EXISTENCE.

>>And the third reason is theoretical. Independent what happens in
>>practice we would like our theory (data model) be general enough to
>>explain very exotic cases. This inclues such extremes as one wide table
>>with all possible columns and many narrow tables with a few columns.
>>

> 
> 
> That is a pretty bizarre argument. The relational model is general
> enough to represent entities with any non-negative number of
> attributes. It doesn't need nulls to do this. The question of
> "explaining" tables with all possible columns or with few columns
> doesn't arise. We model the real world facts - we don't model tables.

The problem is that there exist different equivalent design for each real world problem domain (a set of facts). So the problem is what concrete structure of tables to choose. One schema might be very simple but needs sparse values (a lot of null values). Another schema has many tables. But note again that all of them are equivalent because have the same number of dimensions by means of which we position our objects. The difference is the way how we pack our facts, in what kind of container structure.

What people cannot understand is that we cannot simply disable nulls. It is too simplistic point of view. It is not possible to say that we will not use nulls and that is all. Why? Because the notion of absence exists in almost any data model. We need to know if an object exists or not. If yes, then we get some value. If not then we get null.

Currently the null construct is only a primitive mechanism for dealing with absence. Namely, we use nulls when we want to store the fact of absence of *primitive* values. However, in general case all types of objects---not only primitive values---can be absent. So in a good model the notion of absence has to be formalized by means of a first level construct. It is an illusion (simplification) that we can build a model without a mechanism for expressing the fact of absence.

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