Entity Relationship(ER) Diagram

1. Er. Nawaraj Bhandari
Topic 3
Entity Relationship Modelling 1

2. Entity Relationship(ER) Diagram
 We will learn to develop ER Diagram in this topic.

3. 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.

4. 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.

5. 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

6. Entity Relationship Modelling 1 Topic 3 - 3.6
© NCC Education LimitedV1.0
Notations 1 – UML
(Unified modeling
language)
Student
Student
on
Module
Module
Attributes:
Module Code(PK)
Module Name
Attributes:
Student No(PK)
Student Name
Attributes:
Module Code (FK)
Student No (FK)
0...*
1
1
0...*

7. Entity Relationship Modelling 1 Topic 3 - 3.7
© NCC Education LimitedV1.0
Notations 2 -
CROWS FEET
Module
Student on
Module
Student
Attributes:
Module Code(PK)
Module Name
Attributes:
Student No(PK)
Student Name
Attributes:
Module Code(PK)(FK)
Student No (PK) (FK)

8. Entity Relationship Modelling 1 Topic 3 - 3.8
© NCC Education LimitedV1.0
Notations 3 -
CHEN
Student
Module Module Code
Student No
M
M
Takes

9. Database Schema
 The overall design or structure of the database is called the database
schema.
 Three types:
1. Internal Schema
2. Conceptual Schema
3. External Schema

10. Types of Schema
1. Internal Schema
describes the physical storage structure of database.
2. Conceptual Schema
hides the details of physical storage structures and concentrates
on describing entities, data types, relationships, operations and
constraints
3. External Schema
describes a subset of the database according to the format the
user wants and hides the rest.

11. 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 independence
2. Logical data independence.

12. 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.

13. 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.

14. 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

15. 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.

16. 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.

17. 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

18. 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.

19. Types of Attributes
4. Simple Attribute
It 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 Attribute
It is composed of more than one component. For example,
'address' might have city, state, country, zip code, etc.

20. Types of Attributes
6. Key Attribute
Primary Key is a key attribute.
Not Null and Unique attribute of an entity
At most one attribute in each entity
7. Derived Attribute
The value of the derived attribute can be derived from the values
of other attributes
Age of a person can be derived from the date of birth and current
date i.e. age = current date – date of birth.

21. Types of Attributes
8. Attribute Domain
A 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.

22. 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.

23. 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)

24. 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.

25. 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 Authors
A teacher teaches many class and a
class studies from many teachers.

26. 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

27. ER Diagram

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

29. ER Diagram(Chen)
N 1

30. ER Diagram(CROWS FEET )

31. ER Diagram (Chen)
N
1
N
M

32. ER Diagram(CROWS FEET )
Linking Table(Entity) or
Associative or Composite
entity

33. Attribute Notation (CHEN)
Single Valued Attribute
Multi-valued Attribute
Key Attribute
Derived Attribute
Composite Attribute

34.  An employee has first name, last name and can have more than one
qualifications.
Multi-valued Attribute

35. Key Attribute
 Primary Key
 A department has Primary Key dept_no, department_name and
location
 foreign key is expressed by dotted underline.

36. 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.

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

38. Strong entity
Strong 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:

39. 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.

40. 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.

41. 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.

42. 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

43. Unary(Recursive) relationship
 The number of associated entity is one.
 An entity related to itself is known as recursive relationship.

44. Binary relationship
 In a binary relationship, two entities are involved.
 Consider the example: each department will have many workers.

45. 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.

46. ER Diagram Notation (CROW'S FEET)
Entity Name
attribute namePK
attribute name
attribute name
Entity Name
Entity Relationship
Illustrates an association between two
entities. It has a name which is a verb. It
also has cardinality.
is assigned to

47. 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.

48. Some examples (CROW'S FEET)
courseteacher
teaches
coursestudent
takes

49. 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..N
AuthorBook AuthorBook
11..N1
Category
1
0..N

50. ER Diagram Notation (CROW'S FEET)

51. 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.]

52. 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.]

53. Possible Solution(UML)
0..*
SkillWorker SkillWorker
10..*1
Lodging
0..*
1

54. Possible Solution(CROW'S FEET)

55. 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.

56. 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

57. Possible Solution(CROW'S FEET)

58. Many-to-Many Relationship
 Module having many Students and Students taking many Modules
Taken ByModule Student
0..N 0..N
Attributes
ModuleID(PK)
ModuleName
Attributes
StudentID(PK)
StudentName

59. Many-to-Many Relationship
Module Student
0..N 0..N
ModuleStudent
1 1
Attributes
ModuleID(PK)
ModuleName
Attributes
StudentID(PK)
StudentName
Attributes
ModuleID(FK)
StudentID(FK)

60. 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.

61. 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.

62. Car Hire Company
Model
HireCar
Reservation
Customer
0..N
1
0..N 11 0..N
0..N 1
1
0..N
1 0..N

63.  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

64. Manufacturing Industry(Chen)

65. Manufacturing Industry(UML)

66. Manufacturing Industry (CROWS FEET)

67. 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_name
location

68. 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_name
location
workers
------------------
emp_no (PK)
first_name
last_name
age
job_title
dept_no (FK)

69. Creating Foreign Key

70. 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

71. 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 Campus
1 1 1..*1..*

72. Entity Relationship Modelling 2 Topic 4 - 4.72
© NCC Education LimitedV1.0
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?

73. Fan Traps – The Solution
 Restructuring the model resolves trap (Campus)-1-has-1:N-
(Department)-1-has-1:N-(Staff)
CampusStaff Department
1 1..* 11..*

74. Entity Relationship Modelling 2 Topic 4 - 4.74
© NCC Education LimitedV1.0
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
Staff
Department
We now know in which department Eric works

75. 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

76. 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 PropertyBranch
1...* 0...*0...11

77. Entity Relationship Modelling 2 Topic 4 - 4.77
© NCC Education LimitedV1.0
Chasm Trap - 1
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)
P1 Yap
Mansions
S1
P2 Hill House
P2 Usher
House
S2
Branch
Staff
Property
Which Branch manages
‘Hill House’?

78. Chasm Traps – The Solution
 The solution is to change the structure and represent both
relationships.
Property
Staff
Branch
1...* 0...*0...11
1 0...*

79. 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’

80. 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

81. END OF TOPIC 3
Any Questions