Prepared By Meri Dedania (AITS)

Database Management System

byMeri DedaniaAssistant Professor

MCA department

Atmiya Institute of Technology & Science

Yogidham Gurukul

Rajkot

Prepared By Meri Dedania (AITS)

Chapter -6

Entity Relationship Data Model

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6.2 – The Entity-Relationship ModelWhy E-R data model is introduced?

Database Design of larger Relational Database

Easy to Understand

The basic Notions of E – R data model

1) Entity Set :

Table

2) Relationship Set:

3) Attributes:

Column

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Entity SetWhat is Entity?

An entity is a thing or object in the real world that is

distinguishable from all other objects.

An entity has some set of properties and value for

some set of properties may uniquely identifies.

What is entity set?

An entity set is set of entities of the same type that

share the same properties or attributes.

The individual entities that represents a set are said to

be the extension of entity set.

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Types of Entity(Concrete & Abstract)

Entity may be concrete(Tangible) like a person or a

book or customer or employee.

Or it may be abstract(intangible) like a holiday ,

transaction or loan.

What are Attributes?

An entity is represented by a set of attributes.

Attributes are descriptive properties hold by

each member of entity set.

Values of entity for each of its attributes.

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Examples for each term

A person who are customer of particular bank is entity

of entity set CUSTOMER.

So CUSTOMER is entity set and all persons who are

customers of bank are entities.

Some Properties or Attributes Which are shared by

CUSTOMER are cust_id , cust_name , c_street , c_city.

CUSTOMER is Tangible Entity.

One another example is LOAN entity set contains

information for the loan given by particular bank to

Customer.

LOAN has Attributes like loan_num , amount.

Loan is intangible entity.

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The value of Properties like 1001 for cust_id , Hayes

for cust_name , Main for c_street , Harrison for c_city

Entity set do not need to be disjoint.

Suppose one Entity set EMPLOYEE for information of

employee are in particular bank.

Who is also Customer of bank.

That person will entity of CUSTOMER & EMPLOYEE

both , or none.

The cust_id & loan_num attributes are used to

identify customer and loan uniquely respectively.

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AttributesWhat is Attributes?

Attributes are descriptive properties which hold by

each member of entity set.

Domain or value set

The set of permitted value called Domain or value

set of that attribute.

Example of Domain

Like Domain of cust_name is set of string of a certain

length.

Similarly , loan_num is set of string of the form “L-n”

where n is positive integer.

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An attribute of an entity set is a function that maps

from the entity set into a domain.

Since an entity set have several attributes , each entity

can be described by a set of pairs , one pair for each

attributes of the entity set.

A particular CUSTOMER entity may be describe by the

set (cust_id , 1001) , (cust_name , Hayes),(c_street , Main

) , (c_city , Harrison).

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Types of Attributes :

1) Simple and composite attributes

Which attribute does not divide into subpart is called simple attributes.

For example Cust_name in CUSTOMER does not have subpart.

Which Attributes divided into subparts is called Composite Attributes.

Cust_name divided into First_name , Middle_name , Last_name.

Also we can substitute the composite attribute like address for C_street , C_city.

Note that composite attribute may appear as a hierarchy.

In Composite attribute like Address is divided into street_number , street_name , and appartment_number.

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2) Single – valued and multivalued Attributes.

Which attributes have single value for particular entity is called single valued attribute.

Which attributes have multiple value for particular entity is called multi valued attribute.

For example in LOAN , the loan_num attribute for a specific loan entity refers to only one loan number.

Assume that an entity set EMPLOYEE has attribute like phone_number.

An employee may have zero , one or more phone number.

Another example of dependant_name of an EMPLOYEE would have zero , one or more dependant.

The upper bound and lower bound of multi value can be assign in number.

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3) Derived Attribute

The value of this kind of attribute can be derived

from the values of other related attributes or

entities.

For example , the customer entity set has attribute age

that indicates the customer’s age.

If CUSTOMER has also attribute like Date_of_birth.

We can calculate age from the date_of_birth and the

current date.

So age is derived attribute and date_of_birth is called

based or stored attribute.

The value of a derived attribute not stored but is

computed when required.

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Relationship SetWhat is relationship?

A relationship is an association among several entities.

For example , two entity sets CUSTOMER and LOAN.

The relationship set borrower to denote the association between CUSTOMER and bank loans that the customers have.

The association between entity sets is referred to as participation.

The function that an entity plays in a relationship is called that entity’s role.

The same entity set participates in a relationship set more then once , in different roles is called recursiverelationship set.

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A relationship may also have attributes called

descriptive attributes.

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The relationship set borrower provide an example of

a binary relationship set.

Binary relationship set involves two entity sets.

The number of entity sets that participate in a

relationship set is also the degree of the relationship

set.

For example , a binary relationship set is of degree 2.

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6.3 - Constraints E – R enterprise schema may define certain constraints

which data value must satisfy.

Here we examine mapping cardinalities , key constraints

and participation constraints.

1) 6.3.1- Mapping Cardinalities :

It represents number of entities to which another

entity can be associated via a relationship set.

It is very useful to describe binary relationship

sets.

The mapping cardinality must be one of the

following.

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One to One :

Any entity set A is associated with at most one entity

in B.

And an entity in B is associated at most one entity in A.

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One to Many :

An entity in A is associated with any(zero or more)

number of entities in B.

An entity in B , however , can be associated with at

most one entity in A.

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Many to One :

An entity in A is associated with at most one entity in

B.

An entity in B , however , can be associated with any

number(zero or more)of entity in A.

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Many to many :

An entity in A is associated with any number (zero or

more) of entities in B.

And an entity in B is associated with any number (zero

or more) of entities in A.

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2) 6.3.2- Keys

Entity Sets

Relationship Sets

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3) 6.3.3 - Participation Constraints

If every entity in entity set E participates in at least

one relationship in R. then relationship set R is said

to be total.

If some entity in entity set E participates in

relationship set R then R is said to be Partial.

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6.4 – Entity Relationship Diagrams E – R diagram can express the overall logical

structure of a database graphically.

Diagram consists of the following major components.

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