Re: Why relational division is so uncommon?

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.

Can't argue about terminology, although it is frequently useful to be able to refer to relational division as a join.

> 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.

I wonder how they can transform set containment into string containment. Do they use a user defined function

contains(string1, string2)

which is possibly adapted to strings of comma separated words?

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

One more point about this schema is normalization. I have a de ja vu of 5 NF generalized to accomodate set joins... Received on Fri Apr 27 2007 - 18:38:57 CEST