12. Databases

Question 1

Normalisation

Answer 1

Every attribute is dependent on upon the key; the whole key and nothing but the key.

Why: Eliminate data inconsistencies; minimise data duplication.

If Db is relational it must be normalised.

Question 2

1NF

Answer 2

Removal of any repeating attributes or groups of attributes.

We do this by adding a primary key and making a new table with relations based on the primary key.

Question 3

2NF

Answer 3

Must be in 1NF first.

Every attribute must be dependent on the whole key and not just part of it. (Composite keys)

Otherwise attributes will repeat.

Question 4

3NF

Answer 4

Must be in 2NF first.

Every attribute is dependent on the key; the whole key and nothing but the key.

Question 5

Tips for normalising

Answer 5

1. If you are unsure, check for any obvious duplication.
2. You can often get to 3NF through common sense and use of entity modelling.
3. Many to many relationships must have a link table in between