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INTRODUCTION

E-R model was introduce by P.P CHEN in 1976.

E-R model is an effective and standard method of

communication amongest different designers, programmersand end users.

E-R modeling is a high level conceptual data model developed

to facilitate database design.

It is very useful in mapping the meaning and interaction ofreal world enterprise onto a conceptual schema.


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E-R model provides the following three main semantic

concept to the designer:

ENTITIES: Which are distinct objects in a user enterprise.

RELATIONSHIP :Which are meaningful interaction among the objects

ATTRIBUTES: Which describe the entities and relationships. Each such

attributes is associated with a value set(Also called domain)and can take a value from this value set.

CONSTRAINTS: On the entities, Relationships and attributes.


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ENTITYRELATIONSHIP

MODEL

Based on collection of real world objects or concept called

entities; ex: employee, student

attribute represents properties of entity; s.s.num

relationship represents interaction between entities

overall logical structure represented by E-R diagram

representing entity sets,

relationships, attributes


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ENTITY: REAL-WORLD OBJECT DISTINGUISHABLE FROM OTHER

OBJECTS.

AN ENTITY IS DESCRIBED (IN DB) USING A SET OF ATTRIBUTES.

ENTITY SET: A COLLECTION OF SIMILAR ENTITIES. E.G., ALLEMPLOYEES.

ALL ENTITIES IN AN ENTITY SET HAVE THE SAME SET OF ATTRIBUTES.

(UNTIL WE CONSIDER ISA HIERARCHIES, ANYWAY!)

EACH ENTITY SET HAS A KEY.

EACH ATTRIBUTE HAS A DOMAIN.

REPRESENTATION/SYNTAX:

ENTITY SET REPRESENTED BY RECTANGLE

ATTRIBUTE REPRESENTED BY OVAL

KEY ATTRIBUTE UNDERLINED

ER ModelDefinitions


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RELATIONSHIP: ASSOCIATION AMONG TWO OR MORE ENTITIES.

E.G., DAN TAKES DATABASE COURSE; ATTISHOO WORKS IN

PHARMACY DEPARTMENT.

.RELATIONSHIP CAN ALSO HAVE ATTRIBUTES (THAT APPEARONLY FOR THISRELATIONSHIP SET)

REPRESENTATION/SYNTAX: A DIAMOND SYMBOL

. ATTRIBUTES REPRESENTED BY OVAL (SAME AS BEFORE)

RELATIONSHIP SET:COLLECTION OF SIMILAR RELATIONSHIPS.

. AN N-ARY RELATIONSHIP SET RRELATES N ENTITY SETS E1 ... EN; EACHRELATIONSHIP IN R INVOLVES ENTITIES E1 E1, ..., EN EN

. SAME ENTITY SET COULD PARTICIPATE IN DIFFERENTRELATIONSHIP

SETS, OR IN DIFFERENT ROLES IN SAME SET.

ER ModelBasics (Contd.)


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SYMBOLS OF E-R DIAGRAM

(Entity set)

(Relationship)

(Attributes)

(Link between attributeand entity set )


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SYMBOLS OF E-R DIAGRAM

(Mandatory one) (Mandatory many)

(Optional one) (Optional many)


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. ATTRIBUTES CAN ONLY BE CONNECTED TO ENTITIES OR

RELATIONSHIPS.

. ENTITIES CAN ONLY BE CONNECTED VIA RELATIONSHIPS

.AS FOR THE EDGES, LETS CONSIDER KINDS OF RELATIONSHIPS

AND INTEGRITY CONSTRAINTS

Connectivityinthe E-R Diagram

PROFESSORS COURSESTEACHES


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CUSTOMER

ORDERPRODUCT

CUST_ID CUST_NAME

UNIT_COST

PROD_DESC

PROD_ID

ORD_DATEPROD_ID PROD_QTY

CUST_ADRESS

SUBMITS

REQUESTS

E-R diagram for M/s ABC & Co


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.CAPTURE PROPERTIES OF THE RELATIONSHIP AND ENTITIES

. EVERY ENTITY SET HAS A KEY ATTRIBUTE

. NO TWO ELEMENTS CAN HAVE THE SAME VALUE ON THIS ATTRIBUTE EXAMPLE: STUDENT ID

. DOES EVERY ELEMENT IN THE ENTITY SET APPEAR/PARTICIPATE IN

THERELATIONSHIP ?

. MUST EVERY STUDENT TAKE A COURSE ?

. DEFINE CONSTRAINTS BASED ON PROPERTIES OF THEMAPPING/RELATION BETWEEN ENTITY SETS

Constraints KeyandParticipation


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. BINARY RELATIONSHIPS CAN BE CLASSIFIED AS ONE

TO-ONE, MANY-TO-ONE, ONE-TO-MANY, MANY-TO

MANY. WHAT IS THE TYPE OF MAPPING/RELATION

Propertiesofrelations

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1-TO-1 1-TO-MANY MANY-TO-1 MANY-TO-MANY


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ADVANTAGES OF E-R DATA MODEL

STRAIGHT FORWARD RELATIONAL REPRESENTATION: Having designed

an E-R diagram for a database application, the relational representation

of the data base model becomes relatively straightforward.EASY CONVERSION FOR E-R TO OTHER DATA MODEL: Conversion from

E-R diagram to a network or hierarchical data model can easily be

accomplished.

GRAPHICAL REPRESENTATION FOR BETTER UNDERSTANDING: An E-R

model gives graphical and diagrammatical representation of various

entities, its attributes and relationships between entities. This is turn

helps in the clear understanding of the data structure and in minimizing

redundancy and other problems.


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DISADVANTAGES OF E-R DATA MODEL

NO INDUSTRY STANDARD FOR NOTATION: there is no industry

standard notation for Developing an E-R diagram.POPULAR FOR HIGH-LEVEL DESIGN: The E-R model is especially

popular for high level data base design.


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