[Year 12 IT Apps] 3NF: Request for clarification

Tue Nov 11 09:05:23 EST 2014

Thanks Robert, that's pretty much what I was thinking. My students have a problem with a primary key being more than one column, probably because Access table design really enforces a single column, record number style of design. A logical PK would be along the lines of CUSTID+PHONE i.e. both/all fields of the table but then there would be no datum that was uniquely dependent on the PK unless we add an unnecessary Comments field.

Laurie

From: itapps-bounces at edulists.com.au [mailto:itapps-bounces at edulists.com.au] On Behalf Of Robert Timmer-Arends
Sent: Monday, 10 November 2014 6:00 PM
To: Year 12 IT Applications Teachers' Mailing List
Subject: Re: [Year 12 IT Apps] 3NF: Request for clarification

Hello Laurie

Customer table: primary key = ID; no repeating groups > 1NF; SN & FN are both dependent only on primary key > 2NF; neither SN or FN determine each other (ID is the only determinant) > 3NF

Phone table: first, why have the 'record number'? If it is an actual field then do you mean it to be a unique identifier? In other words, an ID field?
If this is the intention, then you are right: phone number is dependent on the meaningless ID and we have 3NF.
But, that is the same for any ID. Remember that the primary key is the smallest set of columns that uniquely identify a row. The reason we introduce an ID field is that in a table with many columns, the number of columns that make up the primary key could become quite unwieldy. For your Customer table, for example, if you did not have ID, then the primary key would be SN+FN, and those two columns would have to be duplicated into any table you wished to link to the Customer table.
So, for the phone table - assuming it's going to be the 'destination' of a link, why bother with the RecordNo field at all. The primary key would be ID_FK + Phone, and 1NF, 2NF and 3NF are achieved.

I know where you are coming from though - a few months ago I was trying to come up with some simple examples and found that reducing the table to two columns makes things very difficult for 2NF and 3NF. In effect the problems that 2NF and 3NF are trying to fix just disappear so 2NF and 3NF become pointless. The second thing that makes 2NF and 3NF 'disappear' is the use of ID fields because the problems that 2NF and 3NF are trying to fix only crop up when the primary key is made up of two or more columns.

So, examples that are trying to explain 2NF and 3NF need multi-column primary keys and at least one more column than just the primary key.

Regards
Robert T-A

----- Original Message -----
From: Savage, John L<mailto:savage.john.l at edumail.vic.gov.au>
To: Year 12 IT Applications Teachers' Mailing List<mailto:itapps at edulists.com.au>
Sent: Monday, November 10, 2014 3:55 PM
Subject: [Year 12 IT Apps] 3NF: Request for clarification

I've been reading and re-reading material about 3NF and trying to look at it as an average student (which is easy). My problem is pretty simple and best dealt with by two examples.

First, from Wikipedia is the classic example with which I have no problems, the DOB is uniquely dependent on the key field (Winner).
Tournament Winners

Tournament

Year

Winner

Indiana Invitational

1998

Al Fredrickson

Cleveland Open

1999

Bob Albertson

Des Moines Masters

1999

Al Fredrickson

Indiana Invitational

1999

Chip Masterson

Here is my problem
Customer

Phone

ID

SN

FN

RecordNo

ID_FK

Phone

SAV01

Savage

Laurie

1

SAV01

043123456

SMI05

Smith

Sam

2

SAV01

0393062512

3

SMI05

0438111222

4

SMI05

0396541782

5

SMI05

0412345678

Winner Dates of Birth

Winner

Date of Birth

Chip Masterson

14 March 1977

Al Fredrickson

21 July 1975

Bob Albertson

28 September 1968

These are in 3NF but in the Phones Table the phone number is only UNIQUELY dependant on the meaningless record number, which seems at variance with the "key and nothing but the key" motto OR is the key RecordNo+ID_FK.

I'm definitely missing something in the definition aren't I?

Laurie

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