What is Normalization? 1NF, 2NF, 3NF, BCNF Database Example

The inventor of the relational model Edgar Codd proposed the theory of normalization of data with the introduction of the First Normal Form, and he continued to extend theory with Second and Third Normal Form. Later he joined Raymond F. Boyce to develop the theory of Boyce-Codd Normal Form.

The Theory of Data Normalization in SQL server is still being developed further. For example, there are discussions even on 6^th Normal Form. However, in most practical applications, normalization achieves its best in 3^rd Normal Form. The evolution of SQL Normalization theories is illustrated below-

Here you see Movies Rented column has multiple values. Now let's move into 1st Normal Forms:

1NF (First Normal Form) Rules

• Each table cell should contain a single value.
• Each record needs to be unique.

The above table in 1NF-

1NF Example

A primary is a single column value used to identify a database record uniquely.

It has following attributes

• A primary key cannot be NULL
• A primary key value must be unique
• The primary key values should rarely be changed
• The primary key must be given a value when a new record is inserted.

What is Composite Key?

A composite key is a primary key composed of multiple columns used to identify a record uniquely

In our database, we have two people with the same name Robert Phil, but they live in different places.

Hence, we require both Full Name and Address to identify a record uniquely. That is a composite key.

Let's move into second normal form 2NF

2NF (Second Normal Form) Rules

• Rule 1- Be in 1NF
• Rule 2- Single Column Primary Key

It is clear that we can't move forward to make our simple database in 2^nd Normalization form unless we partition the table above.

We have divided our 1NF table into two tables viz. Table 1 and Table2. Table 1 contains member information. Table 2 contains information on movies rented.

We have introduced a new column called Membership_id which is the primary key for table 1. Records can be uniquely identified in Table 1 using membership id

Database - Foreign Key

In Table 2, Membership_ID is the Foreign Key Foreign Key references the primary key of another Table! It helps connect your Tables

• A foreign key can have a different name from its primary key
• It ensures rows in one table have corresponding rows in another
• Unlike the Primary key, they do not have to be unique. Most often they aren't
• Foreign keys can be null even though primary keys can not 

The above problem can be overcome by declaring membership id  from Table2  as foreign key of membership id from Table1

Now, if somebody tries to insert a value in the membership id field that does not exist in the parent table, an error will be shown!

What are transitive functional dependencies?

A transitive functional dependency is when changing a non-key column, might cause any of the other non-key columns to change

Consider the table 1. Changing the non-key column Full Name may change Salutation. Let's move into 3NF

3NF (Third Normal Form) Rules

• Rule 1- Be in 2NF
• Rule 2- Has no transitive functional dependencies

To move our 2NF table into 3NF, we again need to again divide our table.

3NF Example

Below is a 3NF example in SQL database:

Now our little example is at a level that cannot further be decomposed to attain higher normal forms of normalization. In fact, it is already in higher normalization forms. Separate efforts for moving into next levels of normalizing data are normally needed in complex databases.  However, we will be discussing next levels of normalizations in brief in the following.

BCNF (Boyce-Codd Normal Form)

Even when a database is in 3^rd Normal Form, still there would be anomalies resulted if it has more than one Candidate Key.

Sometimes is BCNF is also referred as 3.5 Normal Form.

4NF (Fourth Normal Form) Rules

If no database table instance contains two or more, independent and multivalued data describing the relevant entity, then it is in 4^th Normal Form.

5NF (Fifth Normal Form) Rules

A table is in 5^th Normal Form only if it is in 4NF and it cannot be decomposed into any number of smaller tables without loss of data.

6NF (Sixth Normal Form) Proposed

6^th Normal Form is not standardized, yet however, it is being discussed by database experts for some time. Hopefully, we would have a clear & standardized definition for 6^th Normal Form in the near future...