Re: boolean datatype ... wtf?

On Sep 29, 11:38 am, paul c <toledobythe..._at_oohay.ac> wrote:
> On 29/09/2010 7:45 AM, Bob Badour wrote:
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> > Paul Mansour wrote:
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> >> On Sep 29, 9:46 am, paul c <toledobythe..._at_oohay.ac> wrote:
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> ...
> >>> Aren't A=B and A>B relations?
>
> >> I was under the impression that they are not, even if A and B are
> >> relations. But I may have misintepreted C.J. Date on this.
>
> >> In "Database in Depth" he writes:
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> >> "In Chapter 2, I mentioned the fact that the equality comparison
> >> operator "=" applies to every type. In partitular, therefore it
> >> applies to relation types.... Now I must immediatly explain that these
> >> opeartors are not relational operators as such -- that is they are not
> >> part of the relational algebra-- because their result is a truth
> >> value, not a relation."
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> > An algebra is a set of values and a set of operations closed on that set
> > of values.
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> > Date is saying that comparisons are not part of the relational algebra
> > in the same way that division is not part of the integer algebra because
> > dividing any two arbitrary integers is not necessarily closed on integers.
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> > Consider an expression of the form:
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> > R = f(A,B)
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> > where both A and B are relations and f is some binary operation. If f is
> > natural join, then R is a relation. If f is a comparison, then R is not
> > a relation; it is a boolean.
> > ...
>
> Thanks for the precise example.  It seems to me that Date means your
> last sentence as his starting point.  Nothing 'wrong' with that, but my
> question might be 'why does that have to be so'?  Isn't it just as valid
> to say that R can be one of DEE or DUM for certain f?- Hide quoted text -
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> - Show quoted text -

Regarding "why does that have to be so?"...

The quote from Date above follows and refers to a discussion of the equality operator for types. It appears to suggest that there should be some consistancy in the result of the operator.

Are you suggesting that 5=6 returns DEE as well, or that 5=6 returns a boolean scalar while R1=R2 (where Rx is a relation) returns DEE or DUM? It seems to me that while it is very important to recognize the value of the concepts of DEE and DUM, that attempting to eliminate a boolean type and replace it with DEE and DUM is only going to add complexity. Received on Wed Sep 29 2010 - 18:36:28 CEST