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# Re: Theory Questions concerning MVD and Functional Dependency.....

From: EscVector <Junk_at_webthere.com>
Date: 12 Dec 2006 13:26:29 -0800

Solomon_Man wrote:
> So,
> With all the fun aside.
>
> Can someone in laymans terms please explain to me MVD. Please include
> examples.
>
> I have looked on line and there are many examples but none have
> "clicked".
>
> I may be making this hard, actually I should say I know I am making
> this hard but I would like to understand this. The class at this point
> means nothing but I would really like to fully understand what a MVD is
> and be able to identify one.
>
> Thanks,
> Chris
>
>
> vnr1995_at_gmail.com wrote:
> > The contrapositive of that statement is: If it is not possible for A to
> > determine B, A multidetermines B (TRUE or False)
> >
> > This is true.
> >
> >
> >
> > On Dec 12, 11:45 am, "Solomon_Man" <cmgra..._at_gmail.com> wrote:
> > > > > If A does not multidetermine B, it is still possible for A to determine
> > > > > B. (TRUE OR FALSE).

The question as I read it is "can A determine B even if B is not a set or list?"
Ask yourself, can A have a one-to-one relationship with B? The answer is YES or in your case, TRUE.

If A --->>B does not equal true
Then it is still possible for
A-->B

A multivalued dependency means that If you know one attributes value, you can always determine another attribute's set value. Relational theory uses a double headed arrow to illustrate this relation (A--->->B). This means A determines multiple Bs. If you know your teacher's name, you can determine his/her students. The student list is the "MultiValue". A functional dependency that is not multivalued is a one-to-one relationship. (A--->B). This means that A determines s B. If I know your student ID, I can determine your first_name.

4NF eliminates multiple multivalued dependencies within a table by splitting the table into multiple tables. The classic example for 4NFis Department, job, part where multiple parts are used by multiple departments for multiple jobs. Adding or removing a part to a department or job requires multiple rows to be touched. Breaking this out into a parts table and a jobs table keyed with department fixes this problem. Received on Tue Dec 12 2006 - 15:26:29 CST

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