Entity Relationship DiagramsObjectives

Learn the Elements of the E-R model (entities, attributes, and relationships)

Show how to apply the E-R model for modeling business situations

To develop data modeling skills To interpret E-R diagrams

Entities

Entity is something that can be identified in the user’s work environment

Usually a noun Entities are grouped into classes or sets A particular entity is an instance of the entity

class Entities are usually depicted in an E-R diagram

as a rectangle with the Entity name in capital letters

Attributes

Attributes (sometimes called properties) are characteristics that describe entities

Identifiers are attributes that describe an entity instance

Identifiers may be unique or non-unique Composite identifiers consist of two or more

attributes (ex: phone number: area code and number)

Relationships

Relationships are associations between entities

Relationships are sometimes referred to as “Has-a” or “Is-a”

Three types of binary relationships– One to one– One to Many – Many to Many

Cardinality

The maximum and minimum number of entities that can occur on one side of the relationship.

Entity Relationship Diagrams

There are many diagramming conventions--based on CASE tools (several will be illustrated in class)

Entity--Capital letters in a rectangle Relationships--Diamonds Maximum Cardinality--inside the diamond Minimum Cardinality

– Hash mark to indicated entity must exist– Oval to indicate entity may or may not be required

Attributes

Attributes can be shown as ellipses or in lists--see figure 3-6

Weak Entities

A weak entity is sometimes called a dependent entity

A weak entity cannot exist without another type of entity exists

A special type of weak entity is an ID-dependent entity

See figure 3-7 for examples

Subtype Entities

Some entities contain option sets of attributes or subtypes

Structures of subtypes and supertypes are called generalized hierarchies

The relationship is called a IS-A relationship Subtypes inherit from supertypes The subtypes may be mutually exclusive or not See figure 3-8

Business Rules

Business Rules may or may not be included in an E-R diagram; depending on whether the application should enforce the business rules or the rules be enforced manually