## 3NF – Third Normal Form

Let * R* be the relational schema,

*is in 3NF only if :*

**R**should be in 2NF.**R**should not contain**R***transitive dependencies.*

What is a Transitive Dependency ? |

Let be a relational Schema and R be the attribute sets over X,Y,Z R.If is functionally dependent on XY (X → Y)and is functionally dependent on Y Z (Y → Z)then is transitive dependent on XZ (X → Z) |

#### Removal of Transitive Dependency

If there is any transitive dependency in the relation, then

- Create a separate relation and copy the dependent attribute along with a copy of its determinant. and remove these determinants from the original table.
- Mark dependent attribute as a foreign key in the original relation and Mark dependent attribute as a Primary key in the separate relation

##### Example of 3NF :

Consider the relation **Sup_City(SID, Status, City)** :

The relations SC and CS are in 3NF as they doesn't contain any transitive dependencies.

#### Possibilities of Redundancy in 3NF

However, there is less redundancy in 3NF than in 2 NF, but again 3NF is not free from redundancy.

The possibilities of redundancy in 3NF are :

```
Some Points regarding 3NF :
1. A table is automatically in 3NF if one of the following hold :
(i) If relation consists of two attributes.
(ii) If 2NF table consists of only one non key attributes.
2. If X → A is a dependency, then the table is in the 3NF, if one of the
following conditions exists :
(i) If X is a superkey
(ii) If X is a part of superkey
3. If X → A is a dependency, then the table is said to be NOT in 3NF if
the following :
(i) If X is a proper subset of some key (partial dependency)
(ii) If X is not a proper subset of key (non key)
```

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2NF – Second Normal Form : | BCNF – Boyce-Codd Normal Form |

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