cs2300: file structures and introduction to database...

CS2300: File Structures and

Introduction to Database Systems

Lecture 5: Entity-Relationship Model

Chapter 3

Doug McGeehan

1

2

Overview of Database Design Process

• Two Main Activities

– Database design

– Applications design

• We focus on Database Design

• Applications Design

– Focuses on the programs and interfaces that

access a database

– Generally considered part of software engineering

33

Purpose of ER Model

• Entity-Relational (ER) model– Informal sketch of a database

– A type of conceptual schema

• ER Diagram:a high-levelvisualizationof a database

ER Diagram

• Three main components

– Entities

– Attributes

– Relationships

4

ER Diagram: Entity

• Recall:

– An Entity is a collection of attributes

defining a particular object

• In the ER model:

– Same definition for Entity

– Entity type: the template for an entity

– Entity set: the entity instances

in a database

5

ER Diagram: Entity

• Recall:

– An Entity is a collection of attributes

defining a particular object

• In an ER Diagram:

– A box enclosing an entity type name

6

Student

Student

name phone

ER Diagram: Attribute

• ER diagram representation of a

simple attribute

– An ellipse enclosing an attribute name

7

• Several types of attributes

– Simple (aka Atomic)

– Composite

– Multi-valued

– Derived


8

Student

age

name

first

middle

last

phone

date of

birth


– Key

• Value must be unique for each and every

individual entity

• Can be used to uniquely identify an entity

• ER diagram: underlined


9

Student

SSN


– Key

• Multiple keys permitted, but each must satisfy

uniqueness on its own


10

Student

SSN

Student ID

License

Number


– Composite key

• Provides uniqueness to an entity

by grouping together several attributes

• Combined attributes must be minimal

• i.e. Uniqueness requires every attribute in

group


11

Vehicle

VINRegistration

State

Tag No.

Exercise

• Identify the entities

present in the

scenario hand out

• Identify attributes

for these entities

• Don’t worry about

relationships yet

Exercise

• Are there attributes that suggest an

intuitive relationship between entities?

13

ER Diagram: Relationship

• Relationships associate two or more

entities

• ER diagram:

– A diamond enclosing a relationship name

14

Instructor SectionTEACHES


• Relationship type:

– Instructor TEACHES Section

• Relationship instance:

– “Doug McGeehan” TEACHES “CS23001A”

15



• Constraints

– Rules governing participation of entities

in a relationship

– Typically defined by the mini-world being

represented

– A dog can be a stray (no owner)

or owned by one person

– A person can only have two biological

parents 16


• Constraints: Cardinality ratio

– The maximum number of relationship

instances in which an entity can participate

– “How many sections can an instructor

teach?”

– “How many instructors are teaching one

particular section?”

17




– Instructor:Section would be 1:N

• “One-to-many”

• Each instructor can teach many distinct

sections

• A single section can only be taught by one

instructor

18

Instructor SectionTEACHES1 N



– 1:1 “One-to-One”

– 1:N “One-to-Many”

– N:1 “Many-to-One”

– M:N “Many-to-Many”

19

Exercise

• Who is enrolled in which course?

• What would the cardinality ratio of this

relationship type be?

20

Student CourseENROLLED IN


• Constraints: Participation

– Does an entity’s existence depend on its

relationship to another entity?

21




– Total participation:

• Every student must be enrolled

in at least one course

• Every course needs enrolled students

22




– Partial participation

• Every instructor does not necessarily need to

teach any sections of any courses

• But every section needs an instructor

23


Partial Total


• Relationship roles

– What would you name a member variable

of a C++ class that references another

class?

24


• Relationship roles

– What would you name a member variable

of a C++ class that references another

class?

25Instructor SectionTEACHES

sectioninstructor

Instructor SectionTEACHEScurrently

teachingtaught by


• Recursive relationship

– A relationship with another entity of the

same type

26

Person

RENTS FROM

landlord tenant


• Relationship degree

– The number of entity types

that participate in a relationship type

27


• Relationship degree: Binary

28


currently

teachingtaught by


• Relationship degree: Ternary

– Entities:

• Person

• Beer

• Tavern

– Relationship:

• Some people prefer to drink a particular beer

at a particular bar

• Better prices? Available food pairings?

29


• Relationship degree: Ternary

30

Tavern Beer

Person

PREFERS


• Relationships with Attributes:

• Can this attribute be moved to one of

the participating entity types? 31

Person DogADOPTED

adoption

date

ER Diagram: Weak Entity

Dependent

• An entity type that does not have a key

attribute

– 1. Double box the weak entity type

32

Employee DependentSUPPORTS

Name Age


Dependent


attribute

– 2. Underline partial key with dotted line

33


Name Age

SUPPORTS


Dependent


attribute

– 3. Identifying relationship becomes a

double diamond

34


Name Age

SUPPORTS


Dependent


attribute

– Can a dependent exist without its

associated employee?

– 4. Total participation of the

weak entity

35


Name Age

SUPPORTS


Dependent

• Composite attribute or Weak entity?

– Rule of thumb: weak entity if it participates

in other relationship types

36


Name Age

COVERSInsurance


• Composite attribute or Weak entity?

– Otherwise, composite attribute

37

Employee Dependent

Name Age

Exercise

• Incorporate relationships into our

database scenario’s diagram

38

39

Another Exercise

• We want to elaborate on the model for Beers,

Taverns, and Drinkers:

– A beer has a name, which is its identifier.

– A beer also has a manufacturer.

– A bar has a name, address and license number. Either

address or license number can be used as its identifier.

– A drinker has a name and a customer ID

– Taverns sell some beers.

– Drinkers enjoy some beers.

– Drinkers frequent some taverns

4040

Another Exercise

Drinker IDname

Beer

manfname

SELLS Bars sell somebeers.

ENJOYS

Drinkers enjoysome beers.FREQUENTS

Drinkers frequentsome taverns.

Tavern

name

license

addr

phone

Announcement

• Group formation

– Those not in a group will be grouped at

random

– Random groups should choose their team

leader themselves

– I’ll email random groups for their leader

• Homework #1 is coming soon

41

Announcement

• Quiz #1 is also coming soon

– Same day as homework is due

• Quiz and Homework will cover all

lectures up to the due date

• Closed book, 1 page crib sheet

• 15 minutes

42

43

What’s Next …

• More about Entity-Relationship (ER)

Model (continued)

• Good Design Principles

43

cs2300: file structures and introduction to database...

Documents