Re: NULLs

On Jan 4, 8:49 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
>
> I'm not sure what you mean. Can you please elaborate on the distinction
> between what it means for information to be missing at the conceptural layer
> and the logical layer? As far as I'm concerned, if there is a difference
> between what is the case and what is represented in the database, that is,
> if there is information that could be represented in the database but isn't,
> then that information is missing. If a model has no nulls but there is a
> difference between what is the case and what is represented in the database,
> then at least one relation must be subject to an open world interpretation
> (if there's no row, then you don't know) instead of the closed world
> interpretation (if there's no row, then it ain't so). That it can be that
> you don't know raises the spectre of 3VL, regardless of what layer you're
> at.

The connection between the model and what is being modeled is only in our head. We can consider the model without considering what the state of the real world is. Indeed, we can have a model that doesn't even *have* a corresponding real-world aspect. So when we talk about "missing" information, that's an attribute of the map between the model and reality in our head. It's not an aspect or an attribute of the model. At all.

If we have a set A, and for each member of A we have either zero or one members of set B, then we can do that in a system without nulls, or in a system with nulls. The question is, which way is better? The answer is, the way without nulls is better.

Marshall Received on Sat Jan 05 2008 - 07:08:40 CET