16.  What is an Entity?

It is a ‘thing’ in the real world with an independent existence.


17.  What is an Entity type?

It is a collection (set) of entities that have same attributes.


18.  What is an Entity set?

It is a collection of all entities of particular entity type in the database.


19.  What is an Extension of entity type?

The collections of entities of a particular entity type are grouped together into an entity set.


20.  What is Weak Entity set?

An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.


21.  What is an attribute?

It is a particular property, which describes the entity.


22.  What is a Relation Schema and a Relation?

A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, …, tn). Each tuple is an ordered list of n-values t=(v1,v2, …, vn).


23.  What is degree of a Relation?

It is the number of attribute of its relation schema.


24.  What is Relationship?

It is an association among two or more entities.


25.  What is Relationship set?

The collection (or set) of similar relationships.


26.  What is Relationship type?  

Relationship type defines a set of associations or a relationship set  among a given set of entity types.  


27.  What is degree of Relationship type?

It is the number of entity type participating.