Re: So what's null then if it's not nothing?
Date: 5 Dec 2005 12:44:06 -0800
Message-ID: <1133815446.112084.183640_at_z14g2000cwz.googlegroups.com>
paul c wrote:
> vc wrote:
> > paul c wrote:
> >
> >>vc wrote:
> >
> > [...]
> >
> >>>I am not sure, as I said before, that the data type is so terribly
> >>>important for storing data. TTM does not provide any rationale for the
> >>>must-be-supported data type.
> >>>...
> >
> > [...]
> >
> >>For one thing, the only way, without going outside the relational
> >>operators (eg. without counting returned 'rows' or checking 'return
> >>codes' or 'status codes') to find out if a relation has any true
> >>propositions .
> >
> >
> > I am not sure what you mean by the above. Could you please elaborate ?
>
> R{} = TRUE.
I do not understand the notation. Is it an empty relation, a synonym
for TABLE_DUM ?
> (In TTM, there is an operator called 'IS_EMPTY' which the
> book doesn't exactly define, at least when I search through it, but Hugh
> Darwen was kind enough to confirm when I asked about it that IS_EMPTY(R)
> is equivalent to R{} being equal to TABLE_DUM
It make sense because TABLE_DUM is *defined* as {}. I still do not
undertsand what R{} means.
> the case for TABLE_DEE which TTM sees as just a value equivalent to TRUE.)
>
> >
> >
> >>is to support a value such as 'TRUE'
> >>I believe TTM
> >>effectively considers TRUE to be equivalent to a projection over zero
> >>columns of a non-empty relation. Basically, it seems to be TTM's way of
> >>being able to give an answer within the algebra that supports FOL's
> >>existential quantifier, ie. the result of the question is in fact a
> >>relation.
> >
> >
> > Could you rephrase that and provide an example ? Also, when is the
> > result of a question ( a query ?) not a relation ?
>
> When I don't have a TRUE and FALSE domain for relation values and I ask
> the question are there any rows in R, I must, for example, use some
> implementation 'count' function whose result would likely not be a
> relation
TTM's 'SUMMARIZE' does approximately the same.
>or I could 'SELECT' all columns 'FROM' some row of R and then
> write procedural code that checks some SQL return code and then prints
> or displays the literal 'TRUE'. By contrast, if I simply 'SELECT *no
> columns* from R', the result is 'TRUE' and it was the relational
> operators that gave the answer, not code in some host language.
>
> To some, this may seem quite trivial, but I'm interested in what is
> intrinsic to the RM and what isn't. Mainly because I imagine a day when
> some relational engine is wired directly to a display without a lot of
> mediating code between the user and the db.
>
> (I'll stop now because the more I write about this, the greater danger
> all the words will confuse me.)
>
> p
Received on Mon Dec 05 2005 - 21:44:06 CET