Re: Why relational division is so uncommon?

Vadim Tropashko <vadimtro_invalid_at_yahoo.com> wrote in news:1177691936.963082.35840_at_t39g2000prd.googlegroups.com:

> On Apr 26, 6:45 pm, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

>> My simple point is that you cannot talk about the set containment
>> join if the relation attributes are not sets.  You can say that the
>> set containment join is similar to relational division but saying
>> that it is the same thing would only confuse people.

What you are saying is, essentially, that

(a)
R1 <X,Y>: {<1, a>, <1, b>, <2, c>}

and

R2 <X,Y>: {<1, {a, b}> <2, {c}>}

are 'the same' relation;

(b)
'/' in

R1 / D1 = {<1>}, where D1<Y> = {<a>, <b>}

is 'the same' operation

as 'project/join' in

project(join(R2, D2, D2.Y is_a_subset_of R2.Y), X) = {<1>}, where D2<Y> = <{a, b}>.

Obviously, neither (a) nor (b) is true, but I won't argue with you ;)

>

>> Some medical db developers
>> represent a set of symptoms as a comma delimited string:
>> "headache,nausea,fever".  By doing that,  they can express relational
>> division more efficiently,  in some cases,  than with traditional
>> well- known SQL queries.

Yes, something like that.

>> > PatientDecease(PatientName,DeceaseName) =
>> > PatientSymptom(PatientName,SymptomName) /
>> > DeceaseSymptom(SymptomName, DeceaseName)

How can a 5NF schema accomodate a set join if the set join tables are not even in 1NF ?!

>
>
Received on Fri Apr 27 2007 - 21:33:51 CEST