A normalisation example

Take the following table.
StudentID is the primary key.
Is it 1NF?

No. There are repeating groups (subject, subjectcost, grade)
How can you make it 1NF?

Create new rows so each cell contains only one value
But now look – is the studentID primary key still valid?

No – the studentID no longer uniquely identifies each row
You now need to declare studentID and subject together to uniquely identify each row.
So the new key is StudentID and Subject.

So. We now have 1NF.
Is it 2NF?

Studentname and address are dependent on studentID (which is part of the key)
This is good.
But they are not dependent on Subject (the other part of the key)

And 2NF requires…
All non-key fields are dependent on the ENTIRE key (studentID + subject)

So it’s not 2NF
How can we fix it?

Make new tables
Make a new table for each primary key field
Give each new table its own primary key
Move columns from the original table to the new table that matches their primary key…

Step 1
STUDENT TABLE (key = StudentID)

Step 2
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)

Step 3
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)

Step 3
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)

Step 4 - relationships
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)

Step 4 - cardinality
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
Each student can only appear ONCE in the student table

Step 4 - cardinality
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
Each subject can only appear ONCE in the subjects table

Step 4 - cardinality
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
A subject can be listed MANY times in the results table (for different students)

Step 4 - cardinality
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
A student can be listed MANY times in the results table (for different subjects)

A 2NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
SubjectCost is only dependent on the primary key,
Subject

A 2NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
Grade is only dependent on the primary key (studentID + subject)

A 2NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
Name, Address are only dependent on the primary key
(StudentID)

But is it 3NF?
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
So it is 2NF!

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
Oh oh…
What?

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
HouseName is dependent on both StudentID + HouseColour

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
Or HouseColour is dependent on both StudentID + HouseName

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
But either way,
non-key fields are dependent on MORE THAN THE PRIMARY KEY (studentID)

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
And 3NF says that non-key fields must depend on nothing but the key

A 3NF check
STUDENT TABLE (key = StudentID)
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
WHAT DO WE DO?

Again, carve off the offending fields
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8

A 3NF fix
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8

A 3NF fix
SUBJECTS TABLE (key = Subject)
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
1
8

A 3NF win!
Or…

The Reveal
Before…
After…
RESULTS TABLE (key = StudentID+Subject)
1
1
8
8
1
8
SUBJECTS TABLE (key = Subject)