Path: utzoo!utgpu!cunews!cognos!nigelc
From: nigelc@cognos.UUCP (Nigel Campbell)
Newsgroups: comp.databases
Subject: Entity Models and BCNF
Message-ID: <9129@cognos.UUCP>
Date: 14 Dec 90 03:36:16 GMT
Reply-To: nigelc@cognos.UUCP (Nigel Campbell)
Organization: Cognos Inc., Ottawa, Canada
Lines: 47

This is a followup to the recent thread on Entity Models and 
Normalization from a colleague who is having problems getting
into the net . You can reach him at colim@cognos.UUCP or try 
3132050@mcimail.com which may reach him faster 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Subject: ERMs & Normalization
Status: OR

Using Normalization to check your ERM is of course redundant, 
but only if your ERM is perfect!

In practice people are rarely perfect and shifting to another paradigm to look
at the same problem another way is a standard way of getting the Human
mind to stop overlooking things.

So in practice checking your ERM against Normalization is far from 
redundant, even if you find nothing it proves your design is right
(or at least internally complete & consistent).

ERM's which only carry Entity names don't have enough information to
derive Normalization. The approach we use is to identify the
Attributes & Key for each Entity, then the dependencies between them.

You can then produce a first cut Relation for each Entity and start checking
it's Normalization.  Personally I've never seen the usefulness in using
an automated package at this point. Identifying the Dependencies is the
real Analysis step, checking whether the Relation is now in 3NF, and fixing it
is a trivial exercise.

As a side issue ERM's allow 1 to 1 relationships between things
which are seperate logical business entities, but have the same unique
identifier (could be Employees & Phones in the current phone system),
Normalization would put these in the same relation. I would advocate keeping them 
apart at the Logical level to keep your options open, and consider whether to 
collapse them into one Relation at the Physical Level.


Colin Moden



-- 
Nigel Campbell          Voice: (613) 783-6828                P.O. Box 9707
Cognos Incorporated       FAX: (613) 738-0002                3755 Riverside Dr.
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