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From: lien@osu-eddie.UUCP
Newsgroups: comp.databases
Subject: Re: Normalized Databases
Message-ID: <3407@osu-eddie.UUCP>
Date: Thu, 2-Apr-87 01:18:01 EST
Article-I.D.: osu-eddi.3407
Posted: Thu Apr  2 01:18:01 1987
Date-Received: Sat, 4-Apr-87 09:49:52 EST
References: <43239@beno.seismo.CSS.GOV>
Reply-To: lien@osu-eddie.UUCP (Yao-Nan Lien)
Distribution: na
Organization: The Ohio State University, CIS Dept.
Lines: 39
Keywords: normal form, relational


Normal form is mainly design for update anomalies. Let's  take a
trivial  example. If you design such a relation to store the
grade  of a class:

 SSN, HOMEWORK1, HOMEWORK2, MIDTERM, FINAL, AVE

Whenever you updaate the values of HOMEWORK1, HOMEWORK2, MIDTERM, 
or FINAL, you have to update the  values in AVE at the same time. 
Otherwise, you may get an inconsistent database. A good database
should not assume that every user will follow the rule
carefully. Thus, this database is dangeous.

(Actually, a good database doesn't need  AVE since it can be
caluculated. This is not a good example.)

The problem in this relation  is that AVE is transitively
dependent on the key (SSN). Normalization  theory tells us that it
is  not in 3NF.  If we can design it in 3NF or higher, 
this anomaly can be avoided. This is the point behind
normalization theory.

I don't have the complete knowledge about it. Please consult
Date's book or Ullman's book to get a better picture.

Yao-Nan  Lien

-- 
------------------------------------------------------------
Yao-Nan Lien
Department of Computer and Information Science
Ohio State University
2036, Neil Ave. Mall
Columbus, Ohio 43210-1277
Tel 614 292-5236

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