The association among entities is called a relationship. For example, an employee works_at a department, a student enrolls in a course. Here, Works_at and Enrolls are called relationships.
Relationship Sets
A set of relationships of similar type is called a relationship set. Like entities, a relationship too can have attributes. These attributes are called descriptive attributes.
Mapping Cardinalities
Cardinality defines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set.
One-to-one
One entity from entity set A can be associated with at most one entity of entity set B and vice versa.
One-to-Many −
One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.
Many-to-One −
More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.
Many-to-Many −
One entity from A can be associated with more than one entity from B and vice versa.
