By Andrew Yee. Why 4NF? Performance – We want database to take up less space.

by Andrew Yee

Why 4NF?

• Performance–We want database to take up

less space.

Why 4NF?

• To eliminate the possibility for data anomalies.

Why 4NF?

• To reduce the need for:–inserts, –updates –deletes

Normal Forms ComparisonNormal Form # What you need?

1NF Remove repeating Groups

2NF Remove Partial Dependencies

3NF Remove Transitive Dependencies

BCNF Make sure every determinant is a Candidate Key

4NF Remove nontrivial Multi-Valued Dependencies (MVD)

Non-Trival Multi-Valued Dependencies (MVD)

• Assume X is the Superkey–“Superkey” still depends on FD’s only.

• Y is not a subset of X• X and Y are not, together, all the

attributes.

Recap of 4NF Definition

• No non-trivial multi-valued dependencies- Except on superset of a candidate key- Multi-valued dependencies are functional

dependencies.

• Table must be in BCNF

Recap of 4NF Definition

• No Duplications

• Table must be in BCNF

Example to get 4NF

• Assume the table is in BCNF• The town, Region, and District together form the candidate

key.

From Reference #1

Example to get 4NF

• Decomposed into two tables

From Reference #1

Example 2: Is it in 3NF or BCNF?Restaurant Pizza Variety Delivery Area

A1 Pizza Thick Crust Springfield

A1 Pizza Thick Crust Shelbyville

A1 Pizza Thick Crust Capital City

A1 Pizza Stuffed Crust Springfield

A1 Pizza Stuffed Crust Shelbyville

A1 Pizza Stuffed Crust Capital City

Elite Pizza Thin Crust Capital City

Elite Pizza Stuffed Crust Capital City

Vincenzo's Pizza Thick Crust Springfield

Vincenzo's Pizza Thick Crust Shelbyville

Vincenzo's Pizza Thin Crust Springfield

Vincenzo's Pizza Thin Crust Shelbyville

Pizza Delivery

From Reference #2

Example 2: Is it in 3NF or BCNF?

• The table has no non-key attributes because its only key is {Restaurant, Pizza Variety, Delivery Area}.

• The answer is BCNF.

From Reference #2

Example 2: Is it in 4NF?Restaurant Pizza Variety Delivery Area

A1 Pizza Thick Crust Springfield

A1 Pizza Thick Crust Shelbyville

A1 Pizza Thick Crust Capital City

A1 Pizza Stuffed Crust Springfield

A1 Pizza Stuffed Crust Shelbyville

A1 Pizza Stuffed Crust Capital City

Elite Pizza Thin Crust Capital City

Elite Pizza Stuffed Crust Capital City

Vincenzo's Pizza Thick Crust Springfield

Vincenzo's Pizza Thick Crust Shelbyville

Vincenzo's Pizza Thin Crust Springfield

Vincenzo's Pizza Thin Crust Shelbyville

Pizza Delivery

From Reference #2

Example 2: Is it in 4NF?

• If we assume, that pizza varieties offered by a restaurant are not affected by delivery area, then it does not meet 4NF.

• The answer is NO.

From Reference #2

Why not in 4NF?Restaurant Pizza Variety Delivery Area

A1 Pizza Thick Crust Springfield

A1 Pizza Thick Crust Shelbyville

A1 Pizza Thick Crust Capital City

A1 Pizza Stuffed Crust Springfield

A1 Pizza Stuffed Crust Shelbyville

A1 Pizza Stuffed Crust Capital City

Elite Pizza Thin Crust Capital City

Elite Pizza Stuffed Crust Capital City

Vincenzo's Pizza Thick Crust Springfield

Vincenzo's Pizza Thick Crust Shelbyville

Vincenzo's Pizza Thin Crust Springfield

Vincenzo's Pizza Thin Crust Shelbyville

Pizza Delivery

From Reference #2

Two Non-trivial: MVD

Why not in 4NF?

• The table has two non-trivial multivalued dependencies on the {Restaurant} attribute (which is not a superkey).

• The dependencies are:– {Restaurant} →→ {Pizza Variety}– {Restaurant} →→ {Delivery Area}

From Reference #2

How to “fix it” (4NF)?Restaurant Pizza Variety Delivery Area

A1 Pizza Thick Crust Springfield

A1 Pizza Thick Crust Shelbyville

A1 Pizza Thick Crust Capital City

A1 Pizza Stuffed Crust Springfield

A1 Pizza Stuffed Crust Shelbyville

A1 Pizza Stuffed Crust Capital City

Elite Pizza Thin Crust Capital City

Elite Pizza Stuffed Crust Capital City

Vincenzo's Pizza Thick Crust Springfield

Vincenzo's Pizza Thick Crust Shelbyville

Vincenzo's Pizza Thin Crust Springfield

Vincenzo's Pizza Thin Crust Shelbyville

Pizza Delivery

From Reference #2

Two Non-trivial: MVD

How to “fix it” (4NF)?

From Reference #2

Restaurant Pizza Variety

A1 Pizza Thick Crust

A1 Pizza Stuffed Crust

Elite Pizza Thin Crust

Elite Pizza Stuffed Crust

Vincenzo's Pizza Thick Crust

Vincenzo's Pizza Thin Crust

Restaurant Delivery Area

A1 Pizza Springfield

A1 Pizza Shelbyville

A1 Pizza Capital City

Elite Pizza Capital City

Vincenzo's Pizza Springfield

Vincenzo's Pizza Shelbyville

Varieties By RestaurantDelivery Areas By Restaurant

No Duplications among rows! Yay!

Practice Example1. Is this 3NF or BCNF?2. Is this 4NF? 3. If not, what are the decomposed tables in 4NF?

Restaurant Pizza Variety Delivery Area

A1 Pizza Thick Crust Springfield

A1 Pizza Thick Crust Shelbyville

A1 Pizza Thick Crust Capital City

A2 Pizza Stuffed Crust Springfield

A2 Pizza Stuffed Crust Shelbyville

Pizza Delivery

Practice Example (Answer)1. BCNF 2. NO 3. See Below

Restaurant Pizza Variety

A1 Pizza Thick Crust

A1 Pizza Stuffed Crust

A2 Pizza Stuffed Crust

Restaurant Delivery Area

A1 Pizza Springfield

A1 Pizza Shelbyville

A1 Pizza Capital City

A2 Pizza Springfield

A2 Pizza Shelbyville

Varieties By Restaurant Delivery Areas By Restaurant

No Duplications among rows! Yay!

References1. Website: Database Design: 4th and 5th Normal Forms « Tod means Fox

URL: http://blog.todmeansfox.com/2007/12/04/database-design-4th-and-5th-normal-forms/

2. Website: Fourth normal form - Wikipedia, the free encyclopediaURL: http://en.wikipedia.org/wiki/Fourth_normal_form