Database Normalisation

Data integrity is where data stored in a database can be trusted. It might fail, for example, if a customer can have 2 dates of birth assigned to them.
Normalisation is a process to structure a table so it cannot express redundant information.
Normalised tables are: easier to understand; easier to enhance and extend; and protected from insertion, update and deletion anomalies.
Levels:
- 1st Normal Form (1NF) - minimum normalisation
  - Using row order to convey information is not permitted (e.g. storing in people in age order, instead it should be stored in a separate column)
  - Mixing data types within the same column is not permitted (e.g. having both strings and numbers in a column (e.g. “two” and 2), this isn’t allowed by the DBMS)
  - Having a table without a primary key is not permitted
  - Repeating groups are not permitted (e.g. comma separated list in one column)
- 2nd Normal Form (2NF)
  - Each non-key attribute must depend on the entire primary key. No part key dependencies.
- 3rd Normal Form (3NF)
  - Every ~~non-key~~ attribute in a table should depend on the key, the whole key, and nothing but the key. No transitive dependencies.
- 4th Normal Form (4NF) - often not needed
  - Multivalued dependencies in a table must be multivalued dependencies on the key.
- 5th Normal Form (5NF) - often not needed
  - The table (which must be in 4NF) cannot be describable as the logical result of joining some other tables together.