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(2) Entity Set

Entity set iscollection of entities of same type.
As in Student table, each row is an entity and have same attributes. In other words we can say a student
table is an entity type
The types of entity-sets are,
Types of Entities

(a) Strongentity type

(b) Weak entity type

Fig. 3.2.1:Types of entities

Example :
Student entity set contains all students in college database.
(a) Strong entities

Definition : Entity type which has itsown key atributes by which we can identify specific entity uniquely is
called as strong entity type.
Example :
In case of Employee entity any specific employee can be identifed by his Employeeid which is primary key of
employee entity.
Incase of student in class each student identified by unique roll number which is his primary key.
Strong entity type is represented by single rectangle.

EName
Employee_id
Age
Employee
Salary

Fig. 3.2.2: Employee entity

(b) Weak entities
Definition :Entity typewhich cannot form distinct key from their attributes and takes help from corresponding
strong entity iscalled as weakentily lype.
These types of entities are dependent on strong entity for primary key.
For some weak entities we assign virtual primary key. Such virtual primary key of weak entity is called as
'discriminator'.
Weak entitytype is represented by double rectangle.

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Database Systems' 3-3

|Dependent
Example : has,
Employee "dependent"
In case of "Dependent" entity depend on employee entity
entity for primary key. 3.2.3:
Weak
Fig.
3.3 Attributes
attributes.
Introduction knownas
properties are
Each entity has its own properties which describes that entity such databasse.
storedin
ofdata
The attribute value that describes each entity becomes a major part
Employee entity may be described by attributes name, age, phone etc. Notation
Type
Attribute (Simple/Single valued/Stored)

A particular entity will have some value for each of its attributes.
Example :
attribute value is Jayendra.
For an employee of with Employee_id 30, the name
3.3.1 Types Of Attribute
The various types of attribute are used in ER diagrams,
Types of Attributes in ER
Diagrams

(a) Composite Attributes

(b)Multivalued attributes

(c) Derived Attributes

(d) Null atribute

(e) Key attributes

Fig. 3.3.1:Types of attribute are used in ER diagrams

(a) Composite Attributes
subparts.
The attributes which can be divided in multiple
Notation
Type
Composite attribute

attributes.
The divisible attributes are composite

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Database Systems 3-4 Entity Relationship Model

Example :
The Name attribute of Student table can be divided into First_Name and LastName.
First_name
Name
Last_name
Employee

Fig. 3.3.2 :Composite attributes

b) Multivalued Attributes
The attribute having more then one value for a same entity is called as multi-valued attribute.
Type Notation

Multi valued Attribute

Example :
A
Single student can have multiple mobile numbers.
mobile

Employee
Fig. 3.3.3 Multi valued attributes
(c) Derived Attributes

Definition :The value of someattribute can bederived from the value of related stored attribute such attributes
are knouwn as derived attributes.

Type Notation

Derived attribute

Example :
Employee tenure can be calculated from stored attribute Date _of joining' of employee by subtracting it from
today's date.
Tenure Derived attribute

Date of joining Stored attribute

Employee

Fig. 3.3.4 : Derived attributes

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3.3.2 Types of Keys

The various types of keys in ER Diagramsare as follows,

Types of Keys

1. Super Key

2. Composite Key

3. Candicdate Key

4. Primary Key

5. Alternate Key

6. Secondary Key

Fig. 3.3.7:Types of keys

1. Super Key
An attribute or set of attributes that uniquely identifies a single tuple in entity.
There can be more than one super keys in single table
Stud_Passport_Num Licence_ Number, ssn) acts like
Example : In Fig. 3.3.6 combination of (Stud_ID, Stud_name,
a super key.
2. Composite Key
identifies a single tuple in entity.
Any key with more than one attributes that uniquely
so, it is a composite key.
Example: In Fig. 3.3.6 a super key has more than one attribute
3. Candidate Key
attributes is a candidate's key.
Asuper key with minimum number of
No subset of candidate key van be key.
key.
(Stud_Passport_Num, Licence_Number,ssn) acts like a Candidates
Example :ln Fig. 3.3.6 combination of
4. Primary Key
entity.
selected key of strong entity which uniquely identify tuple in entity is a primary key of that
A
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Example: In Fig.3.3.6 combination of
5. Alternate Key
selected as primary key.
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(Stud_Passport_ Num, Licence_Number, ssn) acts like alternate
Example: In Fig. 3.3.6 candidate key
6. Secondary Key
used to access a single tuple in entity.
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Database Systems 3-8 Entity Relationship Model

3.5 Additional Constraints

Relationship Types

A. Mavpng Constraints / Carcinalities

i. One is to one

i. One to many

il. Many lo one

iv. Many to many

B. Participation constraints

i. Total participation

i. Partial participation

C. Degree of Relationship

Binary relationship type

ii. Ternary relationship type

Fig. 3.5.1 : Relationship Types

(A) Mapping Constraints / Cardinalities
Number of entities from each side participating in a relationship set.
Cardinality expresses specific number of entity occurrence of related entity.
Types of mapping constraints
(i) One is to one
In thistype of constraint one tuple in entity is related with only one tuple in other entity.
That is one row in table is related with only one row in other table.
Aassociated with at most one entity in B, B associated with at most one entity in A.
Example :
A
associated with at most one entity in B, B associated with at most one entity in A.
One department can have only one manager.
Every row inDepartment table can be having relationship with only one row in Managers table.
Department Manager

Department Have Manager

A B

(a) One to one mapping (b) Representation in ER diagram

Fig. 3.5.2: One to one mapping
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(ii) One to many entity.
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In this type of with
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Example :
One teacher may teach to many
students. Student table.
Every row in Teacher table can have relationship with many rows in

Student
Teacher Students Teacher Teaches
A

diagram
(a) One to many mapping (b) Representation in ER

Fig. 3.5.3: One to many mapping

(ii) Many to one
In this type of constraint many tuple in entity can be related with only one tuple in other entity.
A
associated with at most one entity in B.
Bassociated with any number of entities in entity A.
Example :
Number of employee works for department.
Multiple rows in Employees table can be related with only one row in Department table.
Employee Department

1
Employee Works_fop
A B
Department

(a) Many toone mapping (b)

Fig. 3.5.4
Representation in ER diagram
(iv) Many to many
In this type of constraint many tuple in entity can be related with multiple tuples in other
Aassociated with any number of entities in entity B. entity.
Bassociated with any number of entities in entity A.

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Book Student

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(a) Many to many mapping (b) Representation in ER diagram

Fig. 3.5.5
Example :
Books in library issued by students.
Multiple rows in Book table can be related with many rows in Student table.
(B) Participation constraints
(0) Total participation
In case of total participation every object in an entity must participate in a relationship.
The total participation is indicated by adark line or double line between entity and relationship.
Example :
Every department must have a manager.

Department Managed_by Manager

Fig. 3.5.6:Total participation

(ü) Partial participation
In case of partial participation more than one object in an entity may participate in arelationship.
The total participation is indicated by a single line between entity and relationship.
Example :
Employees works for department.

Employee Works_for Department

Fig. 3.5.7 :Partial participation

() Degree of Relationship (Binary Vs ternary)
The degree of the relationship type is number of participating entity types.
Types
() Binary relationship type
Arelationship of degree two.

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Example :
Set of subclass (Saving Account, Current Account) are Specialization of super class Account.
Account

Saving_Account Current_Account

Fig. 3.6.2 : Specialization

Notation

The subclass defined in aspecialization is attached by lines to acircle which is connected to super class.
The subset symbol on each line connecting a subclass to circle indicates the direction of super class / subclass
relationship.
Specific attribute
An attributeapplied only to entities of particular subclass is called as specific attribute.
3.6.2 Generalization
Wite a short note on :Generalization. MU - Dec.18, 5 Marks

This is reverse process of specialization or this is bottom up approach of Superclass/subclass relationship.

Definition

Definition : Generalization is a process in which we differentiate among several entity types identifying there
common features and generalizing them to a single super class of which original entity type are special subclass.

Example :
Carand Bike allhaving severalcommon attribute they can generalize to the super class vehicle.
Notation

Adiagrammatic notation to distinguish between generalization and specialization is used in some programming
methodologies.
Arrow pointing to generalized superclass represents generalization.
Arrow pointing to generalized subclass represents specialization.

Tyre Model Company Model

Maxspeed) Owner
Car
BIKE

Average Fuel_type
Owner Average no. of passengers

Fig. 3.6.3 : BIKE entity Fig. 3.6.4: Car entity

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Database Systems
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3.6.3
The
Attribute inheritance
attributes of higher and lower level entities created by specialization and generalizations are attribute;

inheritance.
Abstraction through which relationship (aggregation) is treated as higher level entíties.
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VEHICLE
Average

Tyre (Model Company (Model

Owner
BIKE Maxspeed) Car

Average Fuel_type
Owner Average <no. of passengers

Fig. 3.6.5 :Generalized VEHICLE entity

3.6.4
Constraintsand Characteristics of Specialization and Generalization
Constraínts and Characterístics
of Specialization and Generalization

1. Predicate defined subclass

2. Attribute defined specialization

3. User defined subclass

4. Other constraints specialization

a. Disjointness

i. Disjoíntness constraints

ii. Overlapping constraints

b. Completeness constraints

i. Total special1zation

Hü Partial specialization

ii, Posible constraints

Fig. 3.6.6

(1) Predicate defined subclass

exactly which entities will become
In specialization sometimes we can find member of specific subclass by
placing a condition (or apredicate) on the value of some attributes of superclass, such subclasses are called as
predicate subclasses.
Example:
condition job_type = 'typist'
Employee has attribute job_type. We can put
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Database Systems' 3-14 Entity Relationship Model
name
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emp-id (Job_type
Employe
job_type= job_type=Engineer"
"Secretary"
Typing_speed Post
Tgad job_type
Technician"
Secretary Technical Engineer

Fig. 3.6.7 : Predicate defined specialization

(2) Atribute defined specialization
If all subclasses in a specialization have their membership condition on some attribute of superclass then
the specialization itself is called as attribute defined specialization.
This kind of attribute is called as defining attribute of specialization.
name (address

(Emp_id job_type
Employee
Job_type
"Secretary"
"Engineer
Typing_speed Post
Tgrade Technician
Secretary Technician Engineer

Fig. 3.6.8: Attribute defined specialization

(3) User defined subclass
a subclass hence user needs
When we do not have a predefined condition for determining membership to
is known as user defined subclass.
to specify condition for such specialization then the subclass
applies the operations
Membership in such a subclass is determined by database user, when database user
to add an entity to subclass.
not by any condition that may be evaluated
Hence membership is specialized for each entity by user and
automatically.
(4) Other constraints specialization
(a) Disjointness
specify that the subclasses of specialization must
(i) Disjointness constraints :Disjointness constraints
most one subclass of specialization.
be disjoint that means entity can be a memberof the
Example :
Assistant, Technician or Engineer only.
New employee can become member of only one subclass like
Attribute defined Specialization determines the disjointness constraints.
should be single valued.
If attribute is used to define the membership then predicate
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Person can become member of subclasses like student.
belonging to more than one subclasses.
Disjoint subciass is indicated by encircled '0'.
(b) Completeness constraints
(i) Total specialization
may be member of at least one of
Atotal specialization constraints specifies that entity in super class
the subclass in the specialization.
Adouble line is used to represent total specialization in EER diagram.
Example :
An employee must belong to salaried employees or hourly employee.
Employee

Salaried Hourty

Fig. 3.6.1O(a) : Total specialization

(i) Partial specialization

An entity can either belong to a subclass or not belongs to any subclass.
single line used to represent partial specialization in EER diagram.
A
Example :
An employee can be Engineer, Secretary or Technician.
Employee

Engineer|Secretary Technician

Fig. 3.6.10(b) : Partial specialization

(iii) Possible constraints

Disjoint, total constraints
Disjoint, partial constraints
Overlapping, total constraints
Overlapping, partial constraints

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Database Systems' 3-18 Entity Relationship Model

2. Book (Publisher id, Title, Category)

(Publishor_id Titlo

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Publisher id Title

Articlo Dato

Topic

Journal (ID)
JID

Journal

5. Subject (SID, Name)

SID Name

Subject

6. Author (Auth id, Name, Address, Mobile)

Auth id Name

Author Address

Mobile

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1. Articles are published in Journal.
2. Publication is written by Author.
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Database Systems 3-20
Entity Relationship Model
(b) Customer entity
Customner name Custoner ld

Customor

(c) Car entity

Car_modol car number

Owner_nane Car

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Car Acc_id

Accidents

(3) Identify allrelationship

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1 N
Insurance company has Customer

b) Customer owns one or more car

M
Customer OWns Car

c) Each car associated with zero or any number of accidents

1
Car record Accidents

(4) Construct ER diagram by merging all above relationships

car_number
Cust id
Car_model
Customer owns car

Cust _name Owner_Name

register for rocorded

Company_ld Com Address)

Insurance Accidents
Company
Company _name Acc id Car

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Problem

Ex. 3.8.4:Draw ER Diagram for banking enterprise. (10 Marks)

Soln.:
Branch_id)
(Br_Loc

1 N
Bank has Branch

1
(B_Jocation
B_code
LOANS
B_Name
LoanNo
Interest
Having Loan Type
works_for Balance N M
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M
N
Post Salary Address 1 A_C Customer
C_Name)
\0gs C_jd Phone
ISA M
Aco _No.
Transactior
Saving Customer
Date_Time
Type Amount
Interest._rate (Initial_Ami

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Publc a ti
 Entity Relationship Model
Database Systems
Draw ER Diagram for University database consisting
3-23 four Entities Student, Department, Class and Facuity.
Ex.3.8.5 :
Student has a unique id, the student can enroll for and has a most one
major.
Faculty mu_
multiple
classes
by only
faculty. Every studen
belong to department and faculty can teach Each classistaught (10 Marks
willget grade for the class he/she multiple classes.
Soln. : has enrolled.
Number
Name Age
Major
Address CID
SID 1
Student Class
enrolls

1
M M

has

gets for
M

1
DID GNo
Grade

Department Major
Marks
1
Major
(DName
M 1
works Faculty Teachers

FID (FName Address

Ex. 3.8.6: Draw an ER diagram for the education database that contains the
information about an in house company
education training scheme. The relevant relations are:
Course (course-no, title)
Offering (course-no, off-no, off-date, location)
Teacher (course-no, off-no, emp-no)
Enrolment (course-no, off-no, stud-no, grade)
Employee (emp-no, emp-name, job)
Student (stud-no, stud-name, ph-no)
Soln. : (10 Marks)
emp_no Employee

Teacher

off_no
has Course_ no
1 1
Ottering has
Course
$tud ng Student <Enroll

Grade

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 Database Systems' 3-24 Entity Relationship Model

Review Questions

a. 1 Explain ER Diagrams and its components.

o.2 Explain the term aggregation.
O.3 VWrite a short note on:
(0) Subclass and superclass
() Specialization and generalization
(ii) Type Inheritance
(iv) Weak entity set
Q.4 Wite short note on:Total participation, partial participation.
0.5 Define degree.
Q. 6 Explain different types of attributes in ER Model.
Q.7 What is strong entity? Explain with example.
a.8 What is weak entity ? Explain with example.
Q.9 Explain different types of attributes in ER Model.

Q. 10 Write a note on composite attributes.

Q. 11 Write a note on multivalued attributes.
Q. 12 Define Derived attribute.
Q. 13 Write a note on Key attributes. Explain various type of keys in ER Diagram.
Q. 14 What is relationship set?

DDO