Date: Sun, 30 Dec 2007 12:09:52 -0800 (PST)
On Dec 30, 7:23 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "stevedtrm" <steved..._at_hotmail.com> wrote in message
> >> > If everyone is clear NULLS shouldn't be used, why the debate as to what
> >> > to do about them ?
> >> Because SQL allows NULL and even promotes the idea that NULL solves some
> >> problem instead of introducing many.
> > So everyone is agreed that NULLs shouldn't appear anywhere, and its
> > just a matter of time before NULLS become a legacy problem and a
> > relational language supercedes SQL?
> No, Steve, not everyone is agreed.
> > Are the two solutions I suggested before the widely accepted as
> > resolutions to the two problems NULLs were introduced to eradicate?
> >>Missing information? Then surely there should just be no tuple?
> A row may contain a lot more information that would otherwise be useful.
> Would you tell a potential customer that you won't serve them because you
> don't know everything about them? I think not! Haven't you been asked for
> your e-mail address at a retailer? What would you think if the cashier told
> you to get lost for refusing to supply it?
> >>To indicate that there can be no value? Why not a seperate table with
> >>a boolean value in the non-key column?
> In my opinion, null should only ever be used to indicate "There should be a
> value here, but it hasn't been supplied." That there should be a value or
> that there must be a value are structural issues, and therefore should not
> be subject to interpretation. "No value here" invites interpretation,
> because it begs the question, "Should there have even been a value?"
> Structural information is lost by separating a nullable column into its own
> table: the fact that there should always be a value, even if it hasn't been
> supplied is lost.
I think this could be put better. It is not that there should always be a value, but rather that "a value exists in the real world". However I don't really see what is structurally lost by complete decomposition to eliminate nulls, that can't be remedied via constraints.
> That's not to say that it isn't possible to eliminate
> nulls altoghether, it's just that you need several separate tables instead
> of one: one table for the values that must always be present, one table for
> each nullable column (or set of columns in those cases where whenever one
> value is supplied, another value must also be supplied) that indicates
> whether or not there should be a value (or set of values), and one table
> with a column (or set of columns) for those values that just happen to have
> been supplied.
> Note also that the spectre of 3VL disappears when null indicates only "there
> should be a value here, but it hasn't been supplied." Since there should be
> a value, and only one value, a null can simply be replaced by an exclusive
> disjunction that encompasses all of the values in the attribute's domain.
> So there is no need for 3VL, 2VL suffices. (For infinite domains, a
> quantifier could be used that means instead of "there is...," "there is
> exactly one....")
Received on Sun Dec 30 2007 - 21:09:52 CET