2 entity relationship_model

Entity-Relationship
(E/R) Model
Dr P Sreenivasa Kumar
Professor
CS&E Dept I I T Madras

Prof P Sreenivasa Kumar
Department of CS&E, IITM
2
Entity-Relationship (E/R) Model
Widely used conceptual level data model
• proposed by Peter P Chen in 1970s
Data model to describe the database system at the requirements
collection stage
• high level description.
• easy to understand for the enterprise managers.
• rigorous enough to be used for system building.
Concepts available in the model
• entities and attributes of entities.
• relationships between entities.
• diagrammatic notation.

3
Entities
Entity - a thing (animate or inanimate) of independent
physical or conceptual existence and distinguishable.
In the University database context, an individual
student, faculty member, a class room, a course
are entities.
Entity Set or Entity Type-
Collection of entities all having the same properties.
Student entity set – collection of all student entities.
Course entity set – collection of all course entities.

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Attributes
Each entity is described by a set of attributes/properties.
student entity
StudName – name of the student.
RollNumber – the roll number of the student.
Sex – the gender of the student etc.
All entities in an Entity set/type have the same set of attributes.
Chosen set of attributes – amount of detail in modeling.

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Types of Attributes (1/2)
• Simple Attributes
having atomic or indivisible values.
example: Dept – a string
PhoneNumber – an eight digit number
• Composite Attributes
having several components in the value.
example: Qualification with components
(DegreeName, Year, UniversityName)
• Derived Attributes
Attribute value is dependent on some other attribute.
example: Age depends on DateOf Birth.
So age is a derived attribute.

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Types of Attributes (2/2)
• Single-valued
having only one value rather than a set of values.
for instance, PlaceOfBirth – single string value.
• Multi-valued
having a set of values rather than a single value.
for instance, CoursesEnrolled attribute for student
EmailAddress attribute for student
PreviousDegree attribute for student.
• Attributes can be:
simple single-valued, simple multi-valued,
composite single-valued or composite multi-valued.

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Diagrammatic Notation for Entities
entity - rectangle
attribute - ellipse connected to rectangle
multi-valued attribute - double ellipse
composite attribute - ellipse connected to ellipse
derived attribute - dashed ellipse
EmailAddress
AdmissionYear
Program
Student
RollNumber
StudName
Lname
Fname Mname
Sex
Age DateOfBirth

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Domains of Attributes
Each attribute takes values from a set called its domain
For instance, studentAge – {17,18, …, 55}
HomeAddress – character strings of length 35
Domain of composite attributes –
cross product of domains of component attributes
Domain of multi-valued attributes –
set of subsets of values from the basic domain

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Entity Sets and Key Attributes
• Key – an attribute or a collection of attributes whose value(s)
uniquely identify an entity in the entity set.
• For instance,
• RollNumber - Key for Student entity set
• EmpID - Key for Faculty entity set
• HostelName, RoomNo - Key for Student entity set
(assuming that each student gets to stay in a single room)
• A key for an entity set may have more than one attribute.
• An entity set may have more than one key.
• Keys can be determined only from the meaning of the
attributes in the entity type.
• Determined by the designers

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Relationships
• When two or more entities are associated with each other,
we have an instance of a Relationship.
• E.g.: student Ramesh enrolls in Discrete Mathematics course
• Relationship enrolls has Student and Course as the
participating entity sets.
• Formally, enrolls ⊆ Student × Course
• (s,c) ∈ enrolls ⇔ Student ‘s’ has enrolled in Course ‘c’
• Tuples in enrolls – relationship instances
• enrolls is called a relationship Type/Set.

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Degree of a relationship
• Degree : the number of participating entities.
• Degree 2: binary
• Degree 3: ternary
• Degree n: n-ary
• Binary relationships are very common and widely used.

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Diagrammatic Notation for Relationships
Relationship – diamond shaped box
Rectangle of each participating entity is connected by a line to
this diamond. Name of the relationship is written in the box.
A
C
B
R

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Binary Relationships and Cardinality Ratio
E1
E2
R
• The number of entities from E2 that an entity from E1 can
possibly be associated thru R (and vice-versa) determines
the cardinality ratio of R.
• Four possibilities are usually specified:
• one-to-one (1:1)
• one-to-many (1:N)
• many-to-one (N:1)
• many-to-many (M:N)
M N

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Cardinality Ratios
• One-to-one: An E1 entity may be associated with at
most one E2 entity and similarly
an E2 entity may be associated with at
most one E1 entity.
• One-to-many: An E1 entity may be associated with
many E2 entities whereas an E2 entity may
be associated with at most one E1 entity.
• Many-to-one: … ( similar to above)
• Many-to-many: Many E1 entities may be associated with a
single E2 entity and a single E1 entity
may be associated with many E2 entities.

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Cardinality Ratio – example (one-to-one)
Teaches
ResidesIn
Professor Course
CourseID NamePhoneName
Sex
Address
Name
RollNo
Address
Student
Hostel
Room
HostelName
RoomNo
1 1
1 1
Credits

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Cardinality Ratio – example (many-to-one/one-to-many)
belongsTo
guides
Professor Department
Name Location
PhoneName
Sex
Address
Name
Sex
Address
Professor
Student
Name RoomNo
1 N
N 1
Phone
Address (one-to-many)
(many-to-one)

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Cardinality Ratio – example (many-to-many)
Student Courseenrolls
Name RollNo
Address
Name
CourseId
Credits
ProfessorSex
Name Phone
Address
SponsoredProject
Name Sponser
Value Duration
Start
Date
End
Date
worksFor
M N
M N

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Participation Constraints
• An entity set may participate in a relation either totally or
partially.
• Total participation: Every entity in the set is involved in
some association (or tuple) of the relationship.
• Partial participation: Not all entities in the set are involved
in association (or tuples) of the relationship.
Notation:
E1 E2R
total partial

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Example of total/partial Participation
belongsTo
guides
Professor Department
Name Location
PhoneName
Sex
Address
Name
Sex
Address
Professor
Student
Name RoomNo
1 N
N 1
Phone
Address
one-to-many
(many-to-one)

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Structural Constraints
• Cardinality Ratio and Participation Constraints are together
called Structural Constraints.
• They are called constraints as the data must satisfy them to be
consistent with the requirements.
• Min-Max notation: pair of numbers (m,n) placed on the line
connecting an entity to the relationship.
• m: the minimum number of times a particular entity must
appear in the relationship tuples at any point of time
• 0 – partial participation
• ≥ 1 – total participation
• n: similarly, the maximum number of times a particular entity
can appear in the relationship tuples at any point of time

21
Comparing the Notations
E1 E2R
E1 E2R
N 1
(1,1) (0,N)
is equivalent to

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Attributes for Relationship Types
Relationship types can also have attributes.
properties of the association of entities.
Student Courseenrolls
Grade
M N
grade gives the letter grade (S,A,B, etc.) earned by
the student for a course.
neither an attribute of student nor that of course.

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Attributes for Relationship Types – More Examples
belongsToProfessor Department
joinDate
N 1
SponsoredProjectProfessor worksFor
percentTime
M N

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Recursive Relationships and Role Names
• Recursive relationship: An entity set relating to itself
gives rise to a recursive relationship
• E.g., the relationship prereqOf is an example of a recursive
relationship on the entity Course
• Role Names – used to specify the exact role in which the
entity participates in the relationships
• Essential in case of recursive relationships
• Can be optionally specified in non-recursive cases
Course
prereqOf
prerequisite
course
Role Names

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Weak Entity Sets
Weak Entity Set: An entity set whose members owe their
existence to some entity in a strong entity set.
entities are not of independent existence.
each weak entity is associated with some entity of the
owner entity set through a special relationship.
weak entity set may not have a key attribute.
S
Owner entity
Identifying relationship
Always total
WR
Double wall
box

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Weak Entity Sets - Example
Course Section
has
Section
Name
CourseID
Credits
SectionNo
Year
SemesterNo
RoomNo
ClassTimeProfessor
A popular course may have
several sections each taught
by a different professor and
having its own class room
and meeting times
Partial key:
Uniquely identifies a section
among the set of sections
of a particular course

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Complete Example for E/R schema: Specifications (1/2)
In an educational institute, there are several departments and
students belong to one of them. Each department has a unique
department number, a name, a location, phone number and is
headed by a professor. Professors have a unique employee Id,
name, phone number.
We like to keep track of the following details regarding students:
name, unique roll number, sex, phone number, date of birth,
age and one or more email addresses. Students have a local
address consisting of the hostel name and the room number.
They also have home address consisting of house number,
street, city and PIN. It is assumed that all students reside in the
hostels.

28
Complete Example for E/R schema: Specifications (2/2)
A course taught in a semester of the year is called a section. There
can be several sections of the same course in a semester; these
are identified by the section number. Each section is taught by a
different professor and has its own timings and a room to meet.
Students enroll for several sections in a semester.
Each course has a name, number of credits and the department that
offers it. A course may have other courses as pre-requisites i.e,
courses to be completed before it can be enrolled in.
Professors also undertake research projects. These are sponsored
by funding agencies and have a specific start date, end date and
amount of money given. More than one professor can be
involved in a project. Also a professor may be simultaneously
working on several projects. A project has a unique projectId.

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StudentName
RollNo
Address
Street
City
HNo
LocalAddress
HostelName
RoomNo
EmailId
Age
DateOfBirth
Entities - Student
PIN
Sex

30
Entities – Department and Course
Department
Name
Location
Phone
HOD
Course
CourseID
Credits
Name
DeptNo

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Professor
Name
ProfID
PhoneNumber
Project Sponsor
Amount
EndDateStartDate
Section
ClassRoomSectionID
Entities – Professor, Project and Sections
Timing
ProjectId

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E/R Diagram showing relationships
Student Department
Course Professor
works
On
Project
hasSection
Section
prerequisite
Of
teaches
enrolls works
For
belongs
To
N 1
N
N
N
N
N
N
M
M M
1
1
1
offers
1
N

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Design Choices: Attribute versus Relationship
• Should offering department be an attribute of a course or
should we create a relationship between Course and Dept
entities called, say, offers ?
• Later approach is preferable when the necessary entity,
in this case the Department, already exists.
• Should class room be an attribute of Section or
should we create an entity called ClassRoom and
have a relationship, say, meetsIn,
connecting Section and ClassRoom?
• In this case, the option of making classRoom as an attribute
of Section is better as we do not want to give a lot of
importance to class room and make it a an entity.

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Design Choices:
Weak entity versus composite multi-valued attributes
• Note that section could be a composite multi-valued attribute
of Course entity.
• However, if so, section can not participate in relationships,
such as, enrolls with Student entity.
• In general, if a thing, even though not of independent existence,
participates in other relationships on its own, it is best
captured as a weak entity.
• If the above is not the case, composite multi-valued
attribute may be enough.

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Ternary Relationships
Relationship instance (c, p, j) indicates that
company c supplies a component p that is made use of by the project j
Company Component
Project
supply
serves uses
canSupply

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Ternary Relationships
(c,p) in canSupply, (j,p) in uses, (c,j) in serves may not together imply (c,p,j) is
in supply. Whereas the other way round is of course true.
Company Component
Project
supply
serves uses
canSupply
The binary
relationships
together do not
convey the
same meaning
as supply

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