Normal Forms

The database community has developed a series of guidelines for ensuring that databases are normalized. These are referred to as normal forms and are numbered from one (the lowest form of normalization, referred to as first normal form or 1NF) through five (fifth normal form or 5NF).

First Normal Form (1NF)
First normal form (1NF) sets the very basic rules for an organized database:
* It contains two-dimensional tables with rows and columns.
* Eliminate duplicative columns from the same table.
* Create separate tables for each group of related data and identify each row with a unique column or set of columns (the primary key).

Second Normal Form (2NF)
Second normal form (2NF) further addresses the concept of removing duplicative data:
* Meet all the requirements of the first normal form.
* Remove subsets of data that apply to multiple rows of a table and place them in separate tables.
* Create relationships between these new tables and their predecessors through the use of foreign keys.

Third Normal Form (3NF)
Third normal form (3NF) goes one large step further:
* Meet all the requirements of the second normal form.
* Remove columns that are not dependent upon the primary key.

Boyce-Codd Normal Form
A table is in Boyce-Codd normal form (BCNF) if and only if, for every one of its non-trivial functional dependencies X ? Y, X is a superkey—that is, X is either a candidate key or a superset thereof.

Fourth Normal Form (4NF)
Finally, fourth normal form (4NF) has one additional requirement:
* Meet all the requirements of the third normal form.
* A relation is in 4NF if it has no multi-valued dependencies.
Remember, these normalization guidelines are cumulative. For a database to be in 2NF, it must first fulfill all the criteria of a 1NF database.

Fifth Normal Form (5NF)
The criteria for fifth normal form (5NF and also PJ/NF) are:
* The table must be in 4NF.
* There must be no non-trivial join dependencies that do not follow from the key constraints. A 4NF table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys.

Domain/key Normal Form (DKNF)
Domain/key normal form (or DKNF) requires that a table not be subject to any constraints other than domain constraints and key constraints.

Sixth Normal Form (6NF)
A table is in sixth normal form (6NF) if and only if it satisfies no non-trivial join dependencies at all. This obviously means that the fifth normal form is also satisfied. The sixth normal form was only defined when extending the relational model to take into account the temporal dimension.