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The Entity-Relationship Data Model

Chapter 2

(Database Design)

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Steps in designing a database

• Analysis: – What information needs to be stored?– What are the relationships between different

components of the stored information?– What is the suitable database structure (or schema)?

• Design the database structure (using a database design language or notation suitable for expressing design)

• Implementation in DBMS once committed to the design

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Steps in picture

Ideas/Problems

DatabaseStructure

DatabaseDesign

DBMS

Analysis Design Implementation

• E/R diagram • Relational model• ODL, ORDL

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Entity-Relationship Model

• Traditional & popular

• Graphical representation

• Three types of elements– Entity set (rectangle)– Attributes (oval)– Relationship (diamond)

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Entity Set

• Collection of similar objects• Similar to a class in the sense of OOP• Entity vs. entity set • Example: Database about movies, their stars, the

studio that producing them, and other aspects of movies.– A movie is an entity– Collection of movies is an entity set– Other entities in this db: ?

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Attributes

• Entity set has associated attributes• Each attribute represents a property of entities

belonging to the entity set• Example:

– The entity set Movies has the associated attribute Title each movie has a title

• Assumption: attributes are atomic values address with two components (e.g. number, street)

cannot be used as an attribute of an entity set

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Relationship

• Connections among entity sets• Represent a relationship between entity sets

– If E and F are two entity sets and R connect the two, then R is a binary relation between E and F, mathematically we write R E x F.

• Example: Movies and Stars are two entity sets and a connection Stars-In between the two the intention: m is related to s means that star s starts in movie m.

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E/R Diagram

• A graph representing entity sets, attributes, and relationships.– Entity set (rectangle)– Attributes (oval)– Relationship (diamond)

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Movies

Stars_in

Owns

title

year

length

film type

Studios

name

address

Stars

name address

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A reading of the previous diagram

• Three entity sets:– Movies [title,year,length,film type]– Stars [name,address]– Studio [name,address]

• Two relationships– (m:n) – Stars_in(Movies,Stars)– (m:1) – Owns(Movies,Studios)

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Instance of an E/R diagram

• Concrete data of a database, whose structure is represented by a E/R diagram, creates a database instance

• In an instance– For each entity set: a particular finite set of entities,

each has values for each attribute– For each relationship connecting the entities E1, E2,

…,En a set of tuples (e1,e2,…,en) each ei is an entity in the entity set of Ei

– Where are the attributes?

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Example• A possible database instance of the previous E/R

diagram (Note: The info is inserted for illustrative purpose

only – it needs not be true in real life)

Basic instinct 1990 150 Drama

Total recall 1989 120 Mystery

Sharon Stone a1

Arnold Schwarzenegger a2

Basic instinct Sharon Stone

Total recall Sharon Stone

Total recall Arnold Schwarzenegger

Movies Stars

Stars_in

Universal Studio

Dream World

Studio

Basic instinct Universal Studio

Total recall Universal Studio

Owns

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A little of math before continuing

• E, F are two sets– R E x F: R is a binary relation from E to F

• R is a set whose member is a pair (e,f) where e is a member of E and f is a member of F

• R could be empty, could be equal the Cartesian product of E and F

– R is many to one relation if for each e in E there is at most one element (e,f) in R

– R is one to one: many to one from E to F and many to one from F to E

– R is many to many: not many to one from E to F and not many to one from F to E

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many-one

one-many

one-one

many-many

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Multiplicity of Binary E/R Relationship

• R is a binary relation from E to F – One to many– One to one– Many to many

• Representing using arrow in the connection between entity set and relationship– Arrow entering an entity set represents the

‘one’ in the above description

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Movies

Stars_in

Owns

title

year

length

film type

Studios

name

address

Stars

name address

many-many

many-one

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Multiway Relationships• Easy to handle in E/R diagram: connect all

related entity sets with the relationship (Note: weakening the relationship (movie,studio))

Movies Contracts

Studios

Stars

Contracts relationship: (studio,star,movie)

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Roles in Relationship

• An entity set can appear more than one times in a relationship

• E/R diagram: labeling the arc connecting the entity set and the relationship

Movies Sequel_of

Original

Sequel

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Another example

Movies Contracts

Studios

Stars

Producing studioStudioof star

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Attributes on Relationships

• How to record the salary of a star in a contract?

Movies Contracts

Studios

Stars

salary

? ??

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Attributes on Relationships

• How to record the salary of a star in a contract? Ans: attribute of Contracts!

Movies Contracts

Studios

Stars

salary

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Movies

title

year

length

film type

Studios

name

address

Stars

name address

Contracts

salary

The complete diagram of the Contracts relationship.Can we live without *?

**

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Movies

title

year

length

film type

Studios

name

address

Stars

name address

Contracts

salary

Salaries

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Eliminating Attributes on Relationship

• Introducing a new entity set whose attributes are the attributes attached to the relationship

• Connecting the new entity set to the relationship, with the arrow pointed to the new entity set

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Converting Multiway Relationship to Binary Relationship

• Useful when the language for defining the database structure does not allow multiway relationship.

• Simple:– Introducing a new entity set representing the

relationship– Introducing binary relation between the new

entity set and the old ones which are connecting to the relationship

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Movies Contracts

Studios

Stars

Producing studioStudioof star

Movies

Studios

Stars

Contracts

Movie_of

Star_of

Producingstudio

Studioof star

Before

After

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Subclasses in the E/R Model

• Subclass: common in real-life

• Represented by the isa relationship

• isa is one-one relationship

• In E/R diagram: – draw as a triangle– no arrows into the entity sets

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Movies

titleyear length film type

CartoonsMurder

Mysteries

isa isaweaponVoices

Stars

At home: read example 2.10 and 2.11.

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Design Principles

• Faithfulness

• Avoiding redundancy

• Simple

• Select the right relationships

• Select the right kind of element

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Faithfulness

• Entity sets and attributes should reflect reality.• Relationships are created only if they make sense

given what we know about the domain/application.

• Example:– Stars_in relationship should be many-many

– Teaches relationship between Courses and Instructors? What? How?

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No Redundancy

• Motto: Stored everything only one!• Space reason (less serious)• Consistency (serious)• Example: Add an attribute studioName to the

entity set Movies while having the relationship Owns between Movies and Studios – consequences:– More space (obvious: studio name stored twice)– Change in ownership of a movie change in the Owns

relationship and the Movies entity set

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Simplicity

• Do only whatever is necessary!

• Example:

Movies Owns Studios

Movies Owns StudiosHoldingsRepresents

is better than

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Right Relationships

• Should we represent every possible relationships? NO: due to space & redundancy requirements get only the necessarily one

• How? Consider the assumptions, identify those that cannot be deduced from or represented by others

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Movies

title

year

length

film type

Studios

name

address

Stars

name address

Contracts

Stars_in

Owns

Assumption: a star can plays in a movie if there is a contract involving the star and the movie Stars_in is redundant

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Movies

title

year

length

film type

Studios

name

address

Stars

name address

Works_for

Stars_in

Owns

Question: Would it make sense to have a relationship Works_for? Depending …

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Right Kind of Element

• Attribute vs. Entity set/Relationship

• Example: address – in the examples so far: attribute– better as an entity with attributes such as street,

number, zip code, state

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Example: What happens if we replace Studios by its two attributes? redundancy, losing

information (if a studio does not own a movie its address is lost)

Movies

Stars_in

Owns

titleyear

length

film type

Studios

name

address

Stars

name address

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When to use attribute for entity set E?

• All relationships connecting to E must have an arrow entering E.

• Attributes for E must collectively identify an entity. If there are more than one attributes then they must not depend on each other.

• No relationship involves E more than one.

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Converting from Entity sets to Attributes

E R F

A B

F

A B

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Converting from Entity sets to Attributes

E R F

A B F

R FA

BF