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Re: extracting information from schemas

From: chris <damoncwk_at_hotmail.com>
Date: Sun, 3 Dec 2000 04:37:56 +0800
Message-ID: <90bmf3$ite8@hkunae.hku.hk>

Dear Sir,

    In fact this is part of my DBMS assignment. My tutor stressed that without setting any attribute to be null and without creating extra table, there's ways to sort our the problem!

Sybrand Bakker <postbus_at_sybrandb.demon.nl> wrote in message news:90bj52$jeeg$4_at_ID-62141.news.dfncis.de...
>
> "chris" <damoncwk_at_hotmail.com> wrote in message
> news:90bgct$1c0m11_at_hkunae.hku.hk...
> > Dear all,
> > If there are 3 relations as shown below:
> > PreferredTutor(name varchar(40), instructor varchar(40))
> > PreferredInstructor(name varchar(40), tutor varchar(40))
> > Assignment(courseTitle varchar(40), instructor varchar(40))
> >
> > A tuple(x,y) is in PreferredTutor if an instructor x prefers a tutor y
 to
> > work on his course.
> > A tuple(x,y) is in PreferredInstructor if a tutor x prefers to work for
 the
> > instructor y.
> >
> > In reality, tutors may not have any preferred instructor to work with.
> > tutors who do not indicate his preferred instructor and is not preferred
 by
> > any instructor will not have his or her name listed in any of the above
 3
> > tables. Suppose new tables are not allowed to be built, how can I ensure
> > that all tutor names appear somewhere in the tables?
> >
> > And how can I find the total number of tutors using only the above 3
 tables?
> > Thx a lot!
> >
> >
> >
> >
> >
>
> Obviously the database design is wrong as is doesn't adhere to the third
> normal form.
> Your requirements are simply impossible without a separate instructor
 table
> and a separate tutor table. Also, according to your own words, there is no
> guarantee at all *all* instructors and *all* tutors appear in any of the
> three tables.
>
> Regards,
>
> Sybrand Bakker, Oracle DBA
>
>
>
Received on Sat Dec 02 2000 - 14:37:56 CST

Original text of this message

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