Key Attribute

Key Attribute

* A key attribute uniquely identifies an entity in the entity set.   
* Underlined 

candidate keys- primary key.

There can be multiple key attributes called candidate keys in a single entity.
A single candidate key is designated as the primary key.

Graphically,

* Primary key is underlined
* NID – candidate key
* Sometimes, a group of attributes make up the key. This is called a composite key. 

* Always, the minimal set of attributes are considered for the key.

* Therefore,
A key is a minimal set of attributes whose values uniquely identify an entity in the set.