Re: NULLs
- From: Marshall <marshall.spight@xxxxxxxxx>
- Date: Fri, 4 Jan 2008 22:08:40 -0800 (PST)
On Jan 4, 8:49 pm, "Brian Selzer" <br...@xxxxxxxxxxxxxxxxxxx> 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
.
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