[Year 12 IT Apps] Third Normal Form

[Matheson, Heath A]

For normalisation I talk about three problems you can have with databases and why they are problems.


1.       Too many “things” in one field, which makes it hard to search and sort.

2.       Data repeated in many rows which means if something changes such as a customer address you need to change it in lots of places.

3.       Tables that contain calculated values which take up processing power and unnecessary storage space if you have a really, really large database.

0NF – has all the problems
1NF – eliminate problem 1 (separate the data)
2NF – eliminate problem 1 and 2 (make multiple tables, add primary keys and link them)
3NF – eliminate all three problems. (eliminate calculated fields, put them in queries and reports)

I think that’s all they need to know based on the VCAA examination reports so far.

Good luck to all your students!
Heath Matheson
Mount Beauty Secondary College

From: itapps-bounces at edulists.com.au [mailto:itapps-bounces at edulists.com.au] On Behalf Of ken price
Sent: Wednesday, 5 November 2014 10:40 AM
To: Year 12 IT Applications Teachers' Mailing List
Subject: Re: [Year 12 IT Apps] Third Normal Form

It's not often that I'd consider normalisation as a source of interesting reading, but this is rather good. Well done Mark - hopefully this will help others, or at least provide a direct example they can use.

Is it OK for it to be re-used elsewhere with acknowledgement?

Ken Price

On Wed, Nov 5, 2014 at 10:20 AM, Mark <mark at vceit.com<mailto:mark at vceit.com>> wrote:
Hello workers.

In case anyone was waiting for the other shoe to drop, let's cover 3NF.

To achieve 3NF:
- You must have already achieved 1NF and 2NF.
- No non-key field may be dependent on another non-key field.

Another way of saying it is that every non-key field in a table must give some information about the primary key rather than any other key in the table. Any field that does not contribute to the description of the primary key must be removed from the table.

For example... take a table. StudentID+SubjectID together are the primary key.  (BTW - this use of multi-field keys is always to be avoided in the real world, but never mind for now.)

Honours is a Boolean field that is True if Mark is A or above, and False otherwise.

StudentID   SubjectID   Mark   Honours
ABC0001     S01         A      True
ABC0001     S02         A+     True
DEF0002     S01         B      False

The Honours field is dependent on the Mark field (i.e. to find the meaning of the Honours field, you need to refer to the Mark field) - but the Mark field is not the table's primary key. i.e. The Honours field describes the mark, not the student+subject.

So, a non-key field (Honours) is dependent on another non-key field (Mark). So it fails 3NF.
To fix it, do the same as we did before to achieve 2NF... break the offending field away into its own table with its own primary key (Mark) and non-key field (Honours) and relate the new table to the existing one using Mark as the link field.

This 3NF scenario looks VERY much like the 2NF before, doesn't it?
The only difference is that 2NF needed a non-key field to relate to the entire set of fields acting as the primary key.
In 3NF, it's actually simpler - a non-key field must not be dependent on another non-key field. In both cases, the fix is the same: table splitting. In my opinion, 3NF should be called 1.5NF because it seems to be more basic that 2NF, but we must all revere the word of Codd.

Always remember the normalisation oath:
The key (1NF), the whole key (2NF) and nothing but the key (3NF), so help me Codd<http://en.wikipedia.org/wiki/Edgar_F._Codd>.

(It's taken me 4 years, but I think I'm finally getting the hang of normalisation).

--

Mark Kelly
mark AT vceit DOT com
http://vceit.com

I love the sound of people's voices after they stop talking.



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--
--
Dr Ken Price MACS CP ACCE Professional Associate.
President, TASITE http://www.tasite.tas.edu.au

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