•Entities

•ERD Modeling

•Identify and Model Entities

•Define Relationships

•Entity Relationship Layout

•ERD Modeling Relationship and Frequencies

•Special Relationship Types

Module 2: Conceptual Data Modeling with ERD

•An entity, like a noun, is a person, place, thing, or event about which the system is to maintain, correlate, and display information.

•An entity must be within the scope of the system; not all nouns are entities.

•Entities versus Associated Information

•Entity = Table

Entities

Instance of an Entity: An entity instance refers to one distinct entity occurrence. (= Record)

Examples:

• Joe Smith, along with his address and so on, is an occurrence of STUDENT.

• Algebra 101 is an instance of COURSE.

Instance of an Entity

Practice: Identify the entities in the following description by circling the nouns. Remember, entities must be within the scope of the system.

“Buffalo Records and CDs has been in business for only three years and we have already expanded our business to five stores. We’ve decided it might be about time to get a computer to keep track of some of these things that we keep losing information on. For instance, just last week when we did our item inventory, I was sure we had a copy of that new Guns and Roses CD, but a customer came in just yesterday looking for it and it was nowhere to be found? Thought we better get a bit more organized.

Anyway, we have 20 employees now. We need to keep track of their names, addresses, phone numbers, social security numbers, and birth dates. We like to send our employees birthday cards, or cards for other special occasions. Like Joe over there - his wife just had a baby yesterday.”

Question: What are some examples of the entities defined above?

Instance of an Entity (continued)

Entities are classified by three types:

Entity Type ExamplesPhysical Student

MachineProduct

Conceptual Cost CenterSectionAccount

Event TrainingPurchasingState Change (Historical)

Question: What are other examples of the above types of entities?

Classifying Entities

•Entity

•Relationship

•Attribute

•Domain

•Modeling Conventions - Entities

ERD Modeling

Entities have singular unique names. They are pictorially represented by a rounded box, as in the following:

Employee

Customer

Order

Employee

Customer

Order

Modeling Conventions - Entities

•Determine the significant nouns.

•Determine a logical name for the entity.

•Write a description for the entity.

Identifying and Modeling an Entity

Defining a Relationship

•Types of Relationships

•Naming Relationships

•Relationships Naming Syntax

A relationship is a two-directional, significant association between two entities, or an entity and itself

Types of RelationshipsPictorially represented by a line between two entities.Three types of relationships exist:•One to One (1:1)•One to Many (1:M)•Many to Many (M:M)

One to One (1:1)

An instance of Entity A relates to one and only one instance of Entity B. Entity B relates to one and only one instance of entity A.

Types of Relationships

Entity A Entity B

Types of Relationships (continued)

An instance of Entity B relates to one or more instances of Entity A.

An instance of Entity A relates to one and only one instance of Entity B.

One to Many (1:M)

Entity BEntity A

Many to Many (M:M)

An instance of Entity A relates to one or more instance of Entity B.

An instance of Entity B relates to one or more instance of Entity A.

Types of Relationships (continued)

Entity BEntity A

SALESALE ITEM for

Made up of

Each end of a relationship must have a name to describe the relationship, an optionality, and a degree.

Appropriate naming helps to understand the relationship, exposing potential problems early on.

Each SALE must be made up of one or more SALE ITEMs.

Each SALE ITEM must be for one and only SALE.

Relationship Naming Syntax

Each ENTITY A [must be/may be] relationship end name [one and only one/one or more] ENTITY B

Naming Relationships

A relationship may not always exist.

Ticket Passenger

Each Passenger may be the holder of one or more Tickets.

Each Ticket must be for one and only one Passenger.

for

Holder of

Conditional Relationships

Read the Relationship in Each Direction

First read left to right:

Ticket Passengerfor

Each Ticket must be for one and only one Passenger.

Then read right to left:

Ticket PassengerHolder of

Each Passenger may be the holder of one or more Tickets.

Conditional Relationships (continued)

In this practice exercise, you will describe a relationship.

Read and define the following relationships:

Student CourseEnrolled in

Attended by

Treatment Patientfor

receive

Practice: Defining a Relationship

*Entity Relationship Layout

Always draw crow’s-feet

pointing up or to the left.

*ERD Modeling Relationships and Frequencies

(1:1) Rare

(1:1) Rare

(1:M) Very Rare

(1:M) Common

(1:M) Rare

(1:M) Very Common

(1:M) Rare

(M:M) Common

(M:M) Common

(M:M) Rare

Reading a Diagram

Entity

Relationship

Diagram

Sale Item

State

Product

Employee

Customer

Sale

identifier of

Located in

The location for

Manager of

Part of

made up of

contained in

Comprised of

Part of

Initiated by

Sold by

The initiator of

Located in

Located of

Managed by

In this practice exercise, you will read a diagram and describe the relationships.Read the following diagram and describe the relationships:

*Special Relationship Types

•Dependent Relationships

•Associative Relationships

•Intersection Entity

Dependent Relationships (1:M)

Treatment Patientfor

receive

1:M relationships are very common.

1:M relationships that are mandatory in both directions are uncommon.

Model multi-level hierarchical data as several 1:M relationships:

City

Country

Continent

within

Made up of

within

Made up of

A One to Many relationship my be represented as 1:M or 1 to M and specifies one or more in one direction and only one in the other direction. 1:M relationships can be called “dependent,” “hierarchical,” or “parent/child.”

Associative Relationships (M:M)

Student CourseEnrolled in

Attended by

Automobile MechanicServiced by

services

Each student may be enrolled in one or more courses.

Each Course may be attended by one or more Students.

M:M relationships are very common.

M:M relationships are usually optional in both directions.

M:M relationships will be “resolved” by using intersection entities because relational models do not directly accommodate them.

A Many to Many, or M:M relationship indicates a degree of one or more in both directions.

Intersection Entity (Associative Entity)

Intersection (or Associative) Entities are created to decompose a M:M into two 1:M relationships, with the Intersection Entity being the “child” of both original “parent” entities.

Service MechanicServiced by

services

Automobile

Performed onreceives

Review•Entities

•ERD Modeling

•Identify and Model Entities

•Define Relationships

•Entity Relationship Layout

•ERD Modeling Relationships and Frequencies

•Special Relationship Types