Re: Why relational division is so uncommon?

On Apr 26, 3:22 pm, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
> Vadim Tropashko <vadimtro_inva..._at_yahoo.com> wrote innews:1177607284.813162.87940@t38g2000prd.googlegroups.com:
>
> > This is an interesting observation as well. Consider a medical
> > database schema:
>
> > PatientSymptom (
> > name string,
> > symptom string
> > )
>
> > DeceaseSymptom (
> > name string,
> > symptom string
> > )
>
> > Would db designers pressed hard to avoid set joins and create a
> > redundant relation
>
> The db designers won't be hard pressed to avoid set containment joins at
> all because the above schema does't have any set valued attributes to
> join on -:)

Don't understand. Certainly patient can have a set of symptoms, and if this set is a superset of some decease, then he got this decease?

> What relation is redundant depends on your point of view. 'Symptom
> string' is an ugly hack to emulate relational division for the original
> table you've specified below.

How about symptom = headache, does this sounds like a legitimate symptom? And having {headache,temperature} is a prerequisite for a decease = flu?

> You surely do not claim that the symptom
> string is a set valued attribute, do you ? Medical db developers usually
> have to maintain both tables for obvious reasons.

Dont understand, again. Are you challenging the following

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

? Received on Fri Apr 27 2007 - 02:54:50 CEST