1. Module 2: Conceptual Data Modeling with ERD
Entities
ERD Modeling
Identify and Model Entities
Define Relationships
Entity Relationship Layout
ERD Modeling Relationship and Frequencies
Special Relationship Types
Objectives
At the end of this module, you will be able to:
Identify entities.
Determine entity relationships.
Name the three types of relationships.
Determine a relationship’s optionality and degree.
Model entities and relationships.
Create relationship grids.

2. Entities
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 versus Associated Information
Entity (Table) Information (Attributes)
======= ====================
Course Course Name
Course Section
Course Start Time
Student Student Name
Address
Phone
What other attributes might be identified for the Student entity?

3. Instance of an Entity
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.
An instance is the same as a record (row) in a table (entity) in the relational database.

4. Instance of an Entity (continued)
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?
Store
Employee
Merchandise (Item Inventory)
Customer

5. Classifying Entities Entities are classified by three types:
Entities can tangible and intangible.
Question: What are other examples of the above types of entities?

6. ERD Modeling Entity Relationship Attribute Domain
Modeling Conventions - Entities
Entity Relationship Diagrams (ERDs) are part of the business model. The diagrams visually represent entities and the business relationships between them.
Entity
A thing of significance, real or imagined, about which information needs to be stored.
Usually derived from nouns in the “interview” notes: Requirements Modeling (Analysis) – To identify what our client wants for the To-Be (future) system.
Relationship
Identifies how one entity relates to, or is associated with, another entity.
Usually derived from verbs in the interview notes.
Attribute
Any single item of information that serves to qualify, identify, classify, or express the state of an entity.
Domain
A set of business validation rules, format constraints or other properties that apply to attributes. A domain may be a:
List of values.
Range of values.
Attributes in the same domain are subject to a common set of validation checks.

7. Modeling Conventions - Entities
Entities have singular unique names. They are pictorially represented by a rounded box, as in the following:
Employee
Employee
Customer
Customer
Order
Order

8. Identifying and Modeling an Entity
Determine the significant nouns.
Determine a logical name for the entity.
Write a description for the entity.
Interview end-users to determine the objects of significance about which they need to maintain and correlate information.
Determining Entities
Determine the nouns.
Are they things of significance? Is there information of interest that needs to be maintained or correlated for the noun?
Determine a logical name for the entity.
Write a description for the entity.
An example may be “A customer has significance as a person or company who purchases or orders goods.”

9. Defining a Relationship
Types of Relationships
Naming Relationships
Relationships Naming Syntax

10. Types of Relationships
A relationship is a two-directional, significant association between two entities, or an entity and itself
Types of Relationships
Pictorially 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 Relationship:
Boeing Commercial Aircraft Division Salesperson is assigned to one and only one customer (one airline company).
Entity A: Customers (airline companies)
Entity B: Salespersons
Entity B
Entity A
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.

11. Types of Relationships (continued)
One to Many (1:M)
Entity A
Entity B
One to Many:
One salesperson has several customers.
Entity A: Customers
Entity B: Salespersons
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.

12. Types of Relationships (continued)
Many to Many (M:M)
Entity A
Entity B
Many to Many:
Entity A: Students
Entity B: Courses
Useful Relationship Names
The following is a partial list of some useful relationship names:
about  subject of
allocated for  used by
applied to  subject of
assigned to  performed by
at  location of
based on  basis for
child of  parent to
classification for  of
covered by  for
defined by  part of definition of
description of  for
for  shown on
initiated by  initiated for
made up of  for
operated by  operated for
ordered by  filled by
owned by  owner of
part of  composed of
part of  associated with
required by  performed on
responsible for  responsibility of
site of  in
source of  based on
subject of  applied to
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.

13. Naming Relationships
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.
for
SALE ITEM
SALE
Useful Relationship Names
The following is a partial list of some useful relationship names:
about  subject of
allocated for  used by
applied to  subject of
assigned to  performed by
at  location of
based on  basis for
child of  parent to
classification for  of
covered by  for
defined by  part of definition of
description of  for
for  shown on
initiated by  initiated for
made up of  for
operated by  operated for
ordered by  filled by
owned by  owner of
part of  composed of
part of  associated with
required by  performed on
responsible for  responsibility of
site of  in
source of  based on
subject of  applied to
Made up of
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

14. Conditional Relationships
A relationship may not always exist.
for
Ticket
Passenger
Holder of
Each Passenger may be the holder of one or more Tickets.
Each Ticket must be for one and only one Passenger.

15. Conditional Relationships (continued)
Read the Relationship in Each Direction
First read left to right:
for
Ticket
Passenger
Each Ticket must be for one and only one Passenger.
Then read right to left:
Ticket
Passenger
Holder of
Each Passenger may be the holder of one or more Tickets.

16. Practice: Defining a Relationship
In this practice exercise, you will describe a relationship.
Read and define the following relationships:
Enrolled in
Student
Course
Attended by
“A student may be enrolled in one or many courses; a course may be attended by one or many students.”
“A patient may receive one or more treatments; Each treatment must be for one and only one Patient.”
Where a treatment is unique to each patient
The relationship between “treatment” and “patient” can be also considered a M:M (many to many) relationship because a treatment (e.g. surgery) can be given for many patients. (Note: This argument was raised by a student, after I finished Module 2 on April 16, She will receive a big extra credit. (^_^) as a reward. I am very pleased to see many of you begin to understand the relational data modeling.)
for
Treatment
Patient
receive

17. *Entity Relationship Layout
Always draw crow’s-feet
pointing up or to the left.
Place larger, more dynamic entities toward the upper left of the diagram. Avoid crossed lines.

18. *ERD Modeling Relationships and Frequencies
(1:1) Rare
(1:M) Very Rare
(1:M) Common
(1:M) Rare
(1:M) Very Common
(M:M) Common
(M:M) Rare

19. Reading a Diagram Entity Relationship Diagram Sale Item Product Sale
In this practice exercise, you will read a diagram and describe the relationships. Read the following diagram and describe the relationships:
Sale Item
identifier of
Part of
Entity
Relationship
Diagram
contained in
made up of
Product
Part of
Comprised of
Managed by
Sold by
Sale
Employee
Initiated by
Located in
Manager of
The initiator of
The location for
Located in
Customer
State
Located of

20. *Special Relationship Types
Dependent Relationships
Associative Relationships
Intersection Entity

21. Dependent Relationships (1:M)
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.”
Treatment
for
Patient
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
within
Made up of
Country
within
Made up of
Continent

22. Associative Relationships (M:M)
A Many to Many, or M:M relationship indicates a degree of one or more in both directions.
Student
Enrolled in
Course
Attended by
Each student may be enrolled in one or more courses.
Each Course may be attended by one or more Students.
Automobile
Serviced by
Mechanic
services
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.

23. 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
Serviced by
Mechanic
services
Performed on
receives
Automobile

24. Review Entities ERD Modeling Identify and Model Entities
Define Relationships
Entity Relationship Layout
ERD Modeling Relationships and Frequencies
Special Relationship Types
1. What three types of entities exist:
2. In what directions should crow’s feet always be drawn (ideally)?
3. What is the most common type of relationship?
4. What can 1:M relationships be called?