# Entity Relationship Model Part-2

Relationship: A relationship is an association among several entities.

Relationship Set: A relationship set is a set of relationships of the same type.

Relationship Type: A relationship type defines a set of associations among entities of the different entity types.

Two Types of Relationship Constraints:

a)Cardinality Ratio(degree of relationship is also called cardinality)

b)Participation Constraint

a)Cardinality Ratio: Specifics the number of relationship instances that an entity can participate in.The possible cardinality ratios are:

b)Participation Constraint: The participation constraint specifies whether the existence of an entity depends on its being relate to another entity via the relationship type.There are two types of participation constraints:

1)Total Participation Constraints(Existence dependency):The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R. This participation is displayed as a double line connection.

2)Partial Dependency: If only some entities in E participate in relationship in R, the participation of entity set E in relationship R is said to be partial.This participation is displayed as a single line connecting.

Extended E-R Features:

1)Specialization:

• Top down design process
• We take higher level entity and add new attributes to it to produce lower level entity.The lower level entities inherit the characteristics of higher level entity.
• In terms of ER diagram, specialization is depicted by a triangle component labeled ISA.

Consider an entity set person, with attributes name, street, and city. A person may be further classiﬁed as one of the following:

customer

employee

2)Generalization:

• Bottom-up design approach
• Union of lower entity types to produce higher entity types.

3)Aggregation:

Aggregration is a process when relation between two entity is treated as a single entity.Here the relation between Student and Course, is acting as an Entity in relation with Subject.