freeCodeCamp/guide/english/computer-science/databases/normal-form/index.md

title
Normal Form

Normal Form

Normalization was first introduced as part of the relational model. It is the process of organizing data tables and columns in a way that reduces redundancies and improves integrity. This can either be done through :

• synthesis : creates a normalized database design based on a known set of dependencies.
• decomposition : takes an existing (insufficiently normalized) database design and improves it based on the known set of dependencies

There are three common normal forms (1st, 2nd and 3rd) plus a rather advanced form called BCNF. They are progressive : in orther to qualify for the 3rd normal form, a database schema must satisfy the rules of the 2nd normal form, and so on for the 1st normal form.

• 1st normal form : The information is stored in a table, each column contains atomic values, and there are not repeating groups of columns. This :
  1. Eliminates repeating groups in individual tables.
  2. Creates a separate table for each set of related data.
  3. Identifies each set of related data with a primary key

Example

A design that violates the 1st normal form, the "telephone" column does not contain atomic values

customer ID	First name	Last name	Telephone
123	Pooja	Singh	555-861-2025, 192-122-1111
789	John	Doe	555-808-9633
456	San	Zhang	(555) 403-1659 Ext. 53; 182-929-2929

One solution would be to have an extra column for each phone number. But then, this will repeat conceptually the same attribute(phone number). Moreover, adding extra telephone number will require reorganizing the table by adding more column.This is definitely not practicle.

Another solution is to have a separate table for the association customer <-> Telephone: This respects the 1st normal form and there can be as many rows per customer as needed.

customer ID	First name	Last name
123	Pooja	Singh
789	John	Doe
456	San	Zhang

customer ID	Telephone
123	555-861-2025
123	192-122-1111
789	555-808-9633
456	(555) 403-1659 Ext. 53
456	182-929-2929

• 2nd normal form : The table is in the first normal form and all the non-key columns depend on the table's primary key. This narrows the table's purpose.

Example

A design that violates the 2nd normal form. The model full name being the primary key, there are other candidate keys like {manufacturer, model}. The "Manufacturer Country" column is dependant on a non-key column (the Manufacturer).

Manufacturer	Model	Model Full Name	Manufacturer Country
Forte	X-Prime	Forte X-Prime	Italy
Forte	Ultraclean	Forte Ultraclean	Italy
Dent-o-Fresh	EZbrush	Dent-o-Fresh EZbrush	USA
Kobayashi	ST-60	Kobayashi ST-60	Japan
Hoch	Toothmaster	Hoch Toothmaster	Germany
Hoch	X-Prime	Hoch X-Prime	Germany

The normalized design would be to split into two tables like the following:

Manufacturer	Manufacturer Country
Forte	Italy
Dent-o-Fresh	USA
Kobayashi	Japan
Hoch	Germany

Manufacturer	Model	Model Full Name
Forte	X-Prime	Forte X-Prime
Forte	Ultraclean	Forte Ultraclean
Dent-o-Fresh	EZbrush	Dent-o-Fresh EZbrush
Kobayashi	ST-60	Kobayashi ST-60
Hoch	Toothmaster	Hoch Toothmaster
Hoch	X-Prime	Hoch X-Prime

• 3rd normal form : The table is in second normal form and all of its columns are not transitively dependent on the primary key. A column is said to be dependant on an another column if it can be derived from it, for example, the age can be derived from the birthday. Transitivity means this dependance might involve other columns. for example, if we consider three columns PersonID BodyMassIndex IsOverweight , the column 'IsOverweight' is transitively dependant on 'personID' through 'BodyMassIndex'.

Example

A design that violates the 3rd normal form. {Tournament, Year} is the primary key for the table and the column 'Winner Date of Birth' transitively depends on it.

Tournament	Year	Winner	Winner Date of Birth
Indiana Invitational	1998	Al Fredrickson	21 July 1975
Cleveland Open	1999	Bob Albertson	28 September 1968
Des Moines Masters	1999	Al Fredrickson	21 July 1975
Indiana Invitational	1999	Chip Masterson	14 March 1977

A design compliant with the 3rd normal form would be :

Tournament	Year	Winner
Indiana Invitational	1998	Al Fredrickson
Cleveland Open	1999	Bob Albertson
Des Moines Masters	1999	Al Fredrickson
Indiana Invitational	1999	Chip Masterson

Winner	Date of Birth
Chip Masterson	14 March 1977
Al Fredrickson	21 July 1975
Bob Albertson	28 September 1968

More Information:

• database normalisation on wikipedia
• first normal form on wikipedia
• second normal form on wikipedia
• third normal form on wikipedia