Entity Relationship Modelling

Er. Nawaraj Bhandari

Topic 3 Entity Relationship Modelling 1

Entity Relationship(ER) Diagram

We will learn to develop ER Diagram in this topic.

Entity

An entity can be defined as anything, which can be uniquely identified and about which data is stored in a database.

A place, person, picture, concept (e.g. course, position, job) are the example of an entity.

Some other examples can be student, workers, car, departments, etc.

Entity Type

An entity type is a collection of similar entities.

An entity-type is a category.

An entity type is a group of objects, people, concepts or other items that have the same set of properties or attributes.

Entity Relationship(ER) Modelling

In 1976, Peter Chen first proposed modeling databases using a graphical technique that humans can relate easily.

An Entity–Relationship (ER) model is an abstract or conceptual way of describing a database

An ER Model is based on a perception of a real world that consists of collection of basic objects called entities and relationships among these objects.

3 Notations Type: UML, CHROWS FEET,CHEN

Entity Relationship Modelling 1 Topic 3 - 3.6

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Notations 1 – UML(Unified modeling language)

Student

Student on

Module

ModuleAttributes:Module Code(PK)Module Name

Attributes:Student No(PK)Student Name

Attributes:Module Code (FK)Student No (FK)

0...*

1

1

0...*

Entity Relationship Modelling 1 Topic 3 - 3.7

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Notations 2 - CROWS FEET

Module

Student onModule

Student

Attributes:Module Code(PK)Module Name

Attributes:Student No(PK)Student Name

Attributes:Module Code(PK)(FK)Student No (PK) (FK)

Entity Relationship Modelling 1 Topic 3 - 3.8

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Notations 3 -CHEN

Student

Module Module Code

Student No

M

M

Takes

Database Schema

The overall design or structure of the database is called the database schema.

Three types:1. Internal Schema2. Conceptual Schema3. External Schema

Types of Schema

1. Internal Schemadescribes the physical storage structure of database.

2. Conceptual Schemahides the details of physical storage structures and

concentrates on describing entities, data types, relationships, operations and constraints

3. External Schemadescribes a subset of the database according to the format

the user wants and hides the rest.

Data Independence

The ability to modify schema definition in one level without affecting schema definition in the next higher level is called data independence.

There are two levels of data independence.1. Physical data independence2. Logical data independence.

Physical Data Independence

Physical data independence means the capacity to change the internal schema or physical storage without affecting conceptual or external schema.

Internal schema may be changed to improve the performance.

Logical Data Independence

Logical data independence is the ability to modify the logical or conceptual schema without affecting to external schema or application program.

Logical Data independence means if we add some new columns or remove some columns from table then the user view and programs should not change.

Entity Relationship(ER) Modelling

The ER model achieves a high degree of data independence which means that the database designer do not have to worry about the physical structure of the database.

The goal of ER Modelling is to produce a model of data that is non-technical and free of ambiguities

Entity Relationship(ER) Diagram

ER diagram is a graphical modelling to standardize ER modelling.

The modeling can be carried out with the help of pictorial representation of entities, attributes, and relationships.

Attributes

An attribute is a property of an entity type.

The data that we want to keep about each entity within an entity type is contained in attributes.

Examples of Attributes :

1. Roll number, name and grade are the attributes of Student.

2. Brand, cost and weight are the attributes of Cellphone.

3. Book name, price, author, publisher, etc. are attributes of Book.

Types of Attributes

1. Single-valued attribute

2. Multi-valued attribute

3. Null Attribute

4. Simple Attribute

5. Composite Attribute

6. Key Attribute

7. Derived Attribute

8. Attribute Domain

Types of Attributes

1. Single-valued attribute : Attribute that holds a single value for an occurrence of an entity type. E.g. age, first_name, last_name, full_name, Gender etc.

2. Multi-valued attribute :Attribute where there might be more than one value for a given occurrence of an entity type, e.g. for the 'Telephone Number', where a person or company might have many of these.

3. Null attribute :Attribute that has NULL.

Types of Attributes

4. Simple AttributeIt is composed of a single component. e.g. age, first_name, last_name, Gender etc. But full_name is not simple attribute. Because it is composed of first_name and last_name.

5. Composite AttributeIt is composed of more than one component. For example, 'address' might have city, state, country, zip code, etc.

Types of Attributes

6. Key AttributePrimary Key is a key attribute.Not Null and Unique attribute of an entityAt most one attribute in each entity

7. Derived AttributeThe value of the derived attribute can be derived from the

values of other attributesAge of a person can be derived from the date of birth and

current date i.e. age = current date – date of birth.

Types of Attributes

8. Attribute DomainA domain is the set of allowable values for an attribute or

number of attributes. A domain therefore limits the values that an attribute can

have. For example, the domain of 'gender' would include the

values 'Male' ,'Female’, ’Other’. Similarly, the domain of fruit would include the values

'Apple', 'Orange' etc.

Relationship

A relationship is an association of entities showing how the entities are related with each other.

The examples of relationship are: Teaching is the relationship between Lecturer and Student. Buying is the relationship between Vendor and Customer. Treatment is the relationship between Doctor and Patient.

Types of Relationship or Multiplicity

Number or range of possible occurrences of an entity type in relation to another entity type (CARDINALITY)

1. One-to-One (1 : 1)

2. One-to-Many (1 : N)

3. Many-to-One (N : 1)

4. Many-to-Many (M : N)

Types of Relationship or Multiplicity

1. One-to -One (1 : 1)Each department can have only one department head.Each worker works in one department.

2. One-to-Many (1 : N)One course can have many students registered.A lecturer may be teaching to many students.

Types of Relationship or Multiplicity

3. Many-to-One (N : 1)Many cities come under one country.Many books can be written by one writer.Many authors can write one book.

4. Many-to-Many (M : N)An Author can write several Books, and a Book can be written by several AuthorsA teacher teaches many class and a class studies from many teachers.

ER Diagram Notation (CHEN)

Represents an entity set, But in CROWS FEET entity set are represents by rounded rectangle.

Represents an attribute

Represent linking of attributes to entity sets and of entity sets to relationship sets.

Represents a relationship

ER Diagram

ER Diagram

Draw an ER Diagram for ClassRoom which has two attributes : room_no and building_name.

ER Diagram(Chen)

N 1

ER Diagram(CROWS FEET )

ER Diagram (Chen)

N

1

N

M

ER Diagram(CROWS FEET )

Linking Table(Entity) or Associative or Composite entity

Attribute Notation (CHEN)

Single Valued Attribute

Multi-valued Attribute

Key Attribute

Derived Attribute

Composite Attribute

An employee has first name, last name and can have more than one qualifications.

Multi-valued Attribute

Key Attribute

Primary Key

A department has Primary Key dept_no, department_name and location

foreign key is expressed by dotted underline.

Derived Attribute

The value of the derived attribute can be derived from the values of other related attributes.

Age of a person can be derived from the date of birth and current date i.e. age = current date – date of birth. In this example, age is the derived attribute.

Composite Attribute

Composite attribute is one which can be further subdivided into simple attributes.

Strong entityStrong entity is one whose existence does not depend on other entity.

Weak entity

Weak entity is an entity that depends on another entity. Weak entity doesn't have key attribute of their own.

Associative or composite entity

Associative entity is used to create many-to-many relationship between different entities

Classification of Entity Sets (CHEN)

Entity sets can be broadly classified into:

Classification of Entity Sets (CHEN)

Consider the example, student takes course. Here student is a strong entity.

Course is considered as weak entity because, if there are no students to take a particular course, then that course cannot be offered. The Course entity depends on the student entity.

Associative or Composite entity

The database relational model does not offer direct support to many-to-many relationships. So, we require associative or composite entity.

Consider the example, customer purchases book.

A customer can purchase many books. And a book can be purchased by many customers. So, here purchases is an associative entity. The entity purchases is mediator for customer and book.

Task : ER Diagram

1. Draw an ER diagram for student.

2. Draw an ER diagram for customers and loan relation.

3. Draw an ER diagram for hospital with a set of patients and medical doctors.

4. Draw an ER diagram for banking system.

5. Draw an ER diagram for car insurance company.

Relationship Degree

Relationship degree refers to the number of associated entities in a relationship.

The relationship degree can be broadly classified into

1. Unary(Recursive) relationship

2. Binary relationship

3. Ternary relationship

Unary(Recursive) relationship

The number of associated entity is one.

An entity related to itself is known as recursive relationship.

Binary relationship

In a binary relationship, two entities are involved.

Consider the example: each department will have many workers.

Ternary relationship

In a ternary relationship, three entities are simultaneously involved.

Example : Consider a project is assigned to many employee. And the project is assigned to one location.

ER Diagram Notation (CROW'S FEET)

Entity Name

attribute namePK

attribute name

attribute name

Entity Name

Entity RelationshipIllustrates an association between two entities. It has a name which is a verb. It also has cardinality.

is assigned to

ER Diagram Notation (CROW'S FEET)

zero or more

one or more

one and only one

zero or 1

Cardinality Cardinality refers to the maximum number of times an instance in one entity can be associated with instances in the related entity. It can be zero or one or more.

Some examples (CROW'S FEET)

courseteacherteaches

coursestudenttakes

ER Diagram Notation (UML)

A Book is written by one or more authors. An author writes one or more Books. A Category may have zero or many books.

Nouns: Book, Author, Category

[Note : Many to Many Relationship cannot be shown in relational model]

1..NAuthorBook AuthorBook

11..N1

Category

1

0..N

ER Diagram Notation (CROW'S FEET)

A Gardening Company

The company hires out workers as they need to.

Workers in the town stay in lodgings. A lodging may have many

workers in it. We are only concerned with the workers’ current

lodging. Workers possess certain skills such as ‘Tree Surgery’

and ‘Garden Design’. They may have more than one skill.

[Note : Try to identify nouns that could possibly be entities.]

A Gardening Company - Nouns

The company hires out workers as they need to.

Workers in the town stay in lodgings. A lodging may have many

workers in it. We are only concerned with the workers’ current

lodging. Workers possess certain skills such as ‘Tree Surgery’ and

‘Garden Design’. They may have more than one skill.

[Note : We are only concerned with one town. So, town is not entity here. But 'Town' is an attribute of the Lodging.]

Possible Solution(UML)

0..*SkillWorker SkillWorker

10..*1

Lodging

0..*

1

Possible Solution(CROW'S FEET)

Equipment & Students

A college department of photography allocates equipment to its students.

A student might be allocated up to 3 types of equipment at a time.

Equipment types can be allocated to many students.

Sports club

A junior sports club keeps track of what sports its members do.

Each member might be involved in more than one sport.

Any sport will have many members involved

Possible Solution(CROW'S FEET)

Many-to-Many Relationship

Module having many Students and Students taking many Modules

Taken ByModule Student0..N 0..N

Attributes

ModuleID(PK)

ModuleName

Attributes

StudentID(PK)

StudentName

Many-to-Many Relationship

Module Student0..N 0..N

ModuleStudent1 1

Attributes

ModuleID(PK)

ModuleName

Attributes

StudentID(PK)

StudentName

Attributes

ModuleID(FK)

StudentID(FK)

Draw the ER : Car Hire Company

Company has a number of models of car available for hire.

Customers hire cars and this is recorded against a particular car rather than a model, as the company might have several cars of the same model.

When a customer wants to reserve a particular model they can do so. This is recorded against a model rather than the car.

When a car of that model becomes available, it will be held for the customer.

When they come in for it, a record is made on the reservation to say which car satisfied the reservation.

Draw the ER : Car Hire Company

Company has a number of models of car available for hire.

Customers hire cars and this is recorded against a particular car rather than a model, as the company might have several cars of the same model.

When a customer wants to reserve a particular model they can do so. This is recorded against a model rather than the car.

When a car of that model becomes available, it will be held for the customer.

When they come in for it, a record is made on the reservation to say which car satisfied the reservation.

Car Hire Company

Model

HireCar

Reservation

Customer0..N

1

0..N 11 0..N

0..N 1

1

0..N

1 0..N

Each supplier has a unique name.

More than one supplier can be located in the same city.

Each part has a unique part number.

Each part has a color.

A supplier can supply more than one part.

A part can be supplied by more than one supplier.

A supplier can supply a fixed quantity of each part.

Manufacturing Industry

Manufacturing Industry(Chen)

Manufacturing Industry(UML)

Manufacturing Industry (CROWS FEET)

Primary Key

The PRIMARY KEY constraint uniquely identifies each record in a database table.

Primary keys must contain unique values.

A primary key column cannot contain NULL values.

Each table should have a primary key, and each table can have only ONE primary key.

departments------------------------dept_no (PK)department_namelocation

Foreign Key

A FOREIGN KEY in one table points to a PRIMARY KEY in another table

A FOREIGN KEY is a column or a combination of columns that is used to establish and enforce a link between two tables or entities.departments------------------------dept_no (PK)department_namelocation

workers------------------emp_no (PK)first_namelast_nameagejob_titledept_no (FK)

Creating Foreign Key

Problems(pit·fall) with ER Models

There are several problems that may arise when designing a ER model. These are known as connection traps.

There are two main types of connection traps:

1. Fan traps

2. Chasm traps

Fan Traps

Fan traps occur in a situation when a model represents relationship between entity types however a path between certain entity occurrences is ambiguous.

Example: (Staff)-1:N-works in-1-(Campus)-1-has-1:N-(Department).

In this model, it may be impossible to determine the department a staff belongs to.

DepartmentStaff Campus1 1 1..*1..*

Entity Relationship Modelling 2 Topic 4 - 4.72

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Fan Traps- 1

Campus ID Campus Name

C1 London

C2 Lagos

C3 Cairo

Staff ID Name Campus ID (FK)

S1 Eric Hudson

C2

S2 Sandra Smith

C1

S3 Malia Shah

C1

Department ID Name Campus ID (FK)

D1 Humanities C1

D2 Computing C2

D3 Maths C2

Campus

Staff Department

In which Department does Eric work?

Fan Traps – The Solution

Restructuring the model resolves trap (Campus)-1-has-1:N-(Department)-1-has-1:N-(Staff)

CampusStaff Department1 1..* 11..*

Entity Relationship Modelling 2 Topic 4 - 4.74

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Fan Traps- 2

Campus ID

Campus Name

C1 London

C2 Lagos

C3 Cairo

Staff ID Name Department ID (FK)

S1 Eric Hudson

D3

S2 Sandra Smith

D1

S3 Malia Shah

D1

Department ID Name Campus ID (FK)

D1 Humanities C1

D2 Computing C2

D3 Maths C2

Campus

StaffDepartment

We now know in which department Eric works

Chasm Traps

Chasm Traps occur where there are relationships between entities, but one of the relationships is non-mandatory.

A model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. This connection traps is known as Chasm Traps.

http://db.grussell.org/section005.html

Chasm Traps – The Problem

A branch has many staff members who manage properties, but not all properties are managed by a member of staff, and not all staff manage properties.

Staff PropertyBranch1...* 0...*0...11

Entity Relationship Modelling 2 Topic 4 - 4.77

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Chasm Trap - 1Branch ID

Branch Name

1 Ilford

2 Redbridge

Staff ID Name Branch ID (FK)

S1 Davinda 1

S2 Roberta 1

S3 Eddie 2

Property ID Property Name

Staff ID (FK)

P1 Yap Mansions

S1

P2 Hill House

P2 Usher House

S2

Branch

Staff

Property

Which Branch manages‘Hill House’?

Chasm Traps – The Solution

The solution is to change the structure and represent both relationships.

PropertyStaff

Branch

1...* 0...*0...11

1 0...*

Chasm Traps – The Solution

Branch ID

Branch Name

1 Ilford

2 Redbridge

Staff ID Name Branch ID (FK)

S1 Davinda

1

S2 Roberta 1

S3 Eddie 2

Property ID

Property Name

Staff ID (FK) Branch ID (FK)

P1 Yap Mansions S1 1

P2 Hill House 2

P3 Usher House S2 1

Branch Staff

Property

We now know which Branch manages ‘Hill House’

References

http://www.studytonight.com/dbms/er-diagram.php

http://www.blackwasp.co.uk/SQLDerivedTables.aspx

http://www2.cs.uregina.ca/~bernatja/crowsfoot.html

http://revj.sourceforge.net/traps.html

http://db.grussell.org/section005.html

http://www.tutorialspoint.com/dbms/dbms_data_independence.htm

http://en.wikipedia.org/wiki/Three_schema_approach

http://en.wikipedia.org/wiki/Data_independence

http://www.tutorialspoint.com/dbms/er_model_basic_concepts.htm

END OF TOPIC 3

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