1. Entity-Relationship Model Ch. 3
Mr John Ortiz

2. Overview of Database Design
Steps in building a database for an application
Requirement analysis. Specify what need to be modeled in a real-world domain
Conceptual design. Specify a high level schema using a database design model
Logical design. Refine a conceptual design to a schema using the data model of a DBMS
Physical design. Define the schema using the DDL of the DBMS
Database creation. Load data into a database
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Entity-Relationship Model

3. Overview of Database Design
Real-world domain
Database design model (ER, OO)
DBMS data model (relational, OO, OR)
Design-to-DBMS step may be automated (e.g., ER-to-relational)
Middle step sometimes skipped (directly to relational model)
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Entity-Relationship Model

4. Entity-Relationship Model
Requirement Analysis
Need to interact with domain experts
Application specific
Time consuming
Very important
Difficult to be thorough
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Entity-Relationship Model

5. Entity-Relationship Model
A Sample Requirement
Consider a regional hospital.
There are doctors, patients, nurses, prescriptions, tests, …
Each doctor has Id, Name, Specialty, Age, Sex, …
Patients see doctors, doctors prescribe drugs to patients, …
Each patient has one primary care physician, a nurse is assigned to at most 5 patients, …
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Entity-Relationship Model

6. Entity-Relationship Model
Conceptual Design
Design a high level schema based on requirements
Why do we do a conceptual design?
To communicate with domain experts (use concepts of the application domain).
It is easier to set up a good model (focus on data content not structure)
Flexible (system independent, hmm… )
How to describe a conceptual design?
Use an ER Diagram (most popular)
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Entity-Relationship Model

7. Entity-Relationship Model
Proposed by Peter Chen in 1976
Historically very popular
Graphical
Major concepts: entity type, relationship type, attribute, constraints, …
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Entity-Relationship Model

8. Entity-Relationship Model
Entity Type
Entity. A real-world object distinguishable from other objects. Each entity is described by a set of attributes (or properties).
Entity Type. A definition of a collection of entities with identical set of attributes.
Students
ssn
name
gpa
 Can informally think of entities as records
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Entity-Relationship Model

9. Entity-Relationship Model
Relationship Type
Relationship. An association between two or more entities (usually of different types). A relationship may also have attributes.
Relationship Type. A definition of a collection of relationships with the identical set of attributes.
gpa
title
credit
cno
grade
name
Enrolled
Courses
Students
ssn
Not every entity participates in a relationship
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Entity-Relationship Model

10. Entity-Relationship Model
Attributes
Domain of an attribute. The set of values the attribute may take.
Many types of attributes. (simple vs composite, single-value vs multi-value, stored vs derived)
Students
ssn
gpa
name
fname
minit
lname
hobby
bdate
age
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Entity-Relationship Model

11. Degree of Relationships
Degree. Number of entity types involved in a relationship type. (unary, binary, ternary, … )
Role. Part played by entities of a unary relationship.
Reports_To
lot
name
subor-dinate
super-visor
ssn
Employees
name
city S#
Suppliers P#
color
Parts
Order
Projects J#
dept
qty
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Entity-Relationship Model

12. Entity-Relationship Model
Key Constraint
Key Attribute(s). A set of attributes of an entity type whose values are unique for each entity of that type.
Key Constraint. Every (almost!) entity type must have a key attribute(s). (e.g., ssn of Students)
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Entity-Relationship Model

13. Participation Constraint
Participation Constraint. Whether or not every entity in an entity type participates in a type of relationship. Either total or partial.
gpa
grade
name
takes
Students
ssn
title
credit
cno
Courses
Every course is taken by some students
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Entity-Relationship Model

14. Cardinality Constraints
Cardinality Ratio. Whether or not an entity of a type may involve in a type of relationship with multiple entities of another type.
One-to-one (1-to-1). Each entity in E1 is associated with 0 or one entity in E2, and vice versa.
Employees
manages
Departments
ssn
dno
name
age
since
room 1
Other notation exists
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Entity-Relationship Model

15. Cardinality Constraints
One-to-many (1-to-m). Each entity in E1 is associated with 0 or more entities in E2, and each entity in E2 is associated with 0 or one entity in E1. The reverse is many-to-one (m-to-1).
Professors
advises
Students
ssn
name
room
gpa 1 m
To determine for Student, fix a Professor, see
how many students are advised by him.
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Entity-Relationship Model

16. Cardinality Constraints
Many-to-many (m-to-n). Each entity in E1 is associated with 0 or more entities in E2, and vice versa.
Students
takes
Courses
ssn
cno
name
gps
title
credit m n
grade
This is the default if not labeled
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Entity-Relationship Model

17. Multiway Relationships
What does it say?
Parts
Suppliers
Projects
order m n r
Pick an entity from
each of the other 2
entity types, how many entities in the third entity type are associated with the pair?
What if r = 1?
What if both r =1 and n=1?
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Entity-Relationship Model

18. Entity-Relationship Model
Weak Entity Type
Weak Entity Type. An entity type without its own key. Typically have a partial key. Must totally participate in an identifying relationship with a strong entity type. The relationship must be m-to-1 or 1-to-1.
Universities
have
Student_clubs
name
cname
city
size
phone
type 1 m
How to refer to a weak entity?
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Entity-Relationship Model

19. Extended ER Model (EER)
Structural Constraint. The min & max number of relationships in R that each entity in E must participate. E
(min, max) R
Students
takes
Courses
ssn
cno
name
gps
title
credit
(1,5)
(5,40)
grade
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Entity-Relationship Model

20. Cardinality ve Structural
Cardinality Structural R E
(0, 1) F 1
(0, m) m
(0, n) n
(1, 1)
The labels are placed in reverse order.
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Entity-Relationship Model

21. Entity-Relationship Model
Look Ahead
Next topic: Relational Data Model
Read from the textbook:
Chapter
Lecture 2
Entity-Relationship Model