The Entity-Relationship (ER) Model

The Entity-Relationship (ER) Model

2.1 Background

·  Used for conceptual database design

·  Relies on concepts of entities, attributes, and relationships

·  Benefits from pictorial representation, called ER diagrams

2.2 An Entity-Relationship (ER) Diagram

2.3 Entity Types

·  Entity types represent sets of objects and are pictured by rectangular nodes

.

An entity set is the collection of instances (i.e., entities) represented by an entity type.

An entity set is also called an extension of the entity type

2.4 Relationship Types

·  Relationship types associate entity types. They are pictured by Diamond nodes, and edges connecting to the related entity types.

A relationship set is the collection of instances (i.e., relationships between objects) represented by a relationship type.

Relationship types may associate an entity type with itself. In such a case, the roles of the entity types in the relationship type are listed on the edges, and the relationship is said to be recursive.

2.5 Attributes

·  Entity types and relationship types might have attributes.

·  The value set, or domain, of an attribute is the set of values that may be assigned to the attribute. Mathematically,

attribute : entity power-set(domain)

·  Atomic attribute types, pictured by oval nodes

·  Composite attribute types, achieved by concatenating simpler attribute types, pictured by trees of atomic attributes

·  Multivalued attribute types

A ‘blue and red’ shirt

·  Derived attribute types displayed in dashed ovals

A ‘age’ from ‘birth date’

2.6 Keys and Weak Entity Types

·  Key attribute types, identify the instances, may be consisted of more than one attribute, displayed with underlined attribute type names

·  Weak entity types have no keys. Displayed by double-rectangular nodes

·  To be identified, instances of weak entity type require an identifying relationship type that relates them to an identifying entity type. Such relations are displayed by double-diamond nodes

·  Weak entity types typically have partial key for distinguishing their instances.

Regular entity types with keys are sometimes called strong entity types.

2.7 Structural Relationship Constraints

Cardinality ratio constraint

·  Specifies the number of relationship instances an entity can participate in

·  Displayed on the diamonds

Participation constraint

·  specifies whether the existence of an entity depends on its being related to another entity via the relationship type

·  Total participation constraints require the participation of every entity the relationship (displayed by double line). Also called existence dependency.

·  Partial participation constraints (displayed by a single line).

Cardinality constraint

Specifies lower and upper bounds on the number of relationships each entity can participate in.

Summary of Notation

Entity /
Weak Entity /
Relationship /
Identifying Relationship /
Attribute /
Key Attribute /
Multivalued Attribute /
Composite Attribute /
Derived Attribute /
Total participation of E2 in R /
Cardinality ratio 1 : N for E1 : E2 in R /
Structural constraint (min,max) on participation of E in R /