Entity Relationship Diagram2

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This presentation was created by me for Database subject. it described the whole process of creating ERD i.e. entity relationship diagrams

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ENTITY RELATIONSHIP DIAGRAM

BY

MEER SADAF NAEEM

Part-1

Understanding the Fundamentals

BASIC CONCEPTS

WHAT IS ENTITY?

Person

WHAT IS ENTITY?

Place

WHAT IS ENTITY?

Object

WHAT IS ENTITY?

Event

ATTRIBUTES

Student

Name Address Class Roll number Percentage

Descriptive properties possessed Descriptive properties possessed by each member of an entity set.by each member of an entity set.

ENTITY SET/TYPE:

Student A Student B Student C Student D

ENTITY SET : STUDENT

Set of entities of the same type Set of entities of the same type that share the same attributes. that share the same attributes.

STUDENT A STUDENT B STUDENT C STUDENT D

Mr. A Mr.DMr. CMr. B

ENTITY SET : STUDENT

ENTITY SET : TEACHER

What is relationship?

Meaningful association Meaningful association among several entities.among several entities.

RELATIONSHIP SET/TYPE:

A set of relationships of the A set of relationships of the same type.same type.

Student A

Student C

Student B

Mr. C

Mr. B

Mr. A

ENTITY SET : STUDENT ENTITY SET : TEACHER

RELATIONSHIP SET : SUBJECT

DEGREE OF RELATIONSHIP SET

The number of entity sets that The number of entity sets that participate in a relationship participate in a relationship set.set.

Student A

Student C

Student B

Mr. C

Mr. B

Mr. A

ENTITY SET : STUDENT ENTITY SET : TEACHER

RELATIONSHIP SET : SUBJECT

TYPES OF ATTRIBUTES

Single and composite:1. roll number is an example of single

attribute as it can not be divided further.

2.Name can be divided further into 3 parts consisting of first name , middle name , last name so it is an example of composite attribute.

TYPES OF ATTRIBUTES

Single valued and multivalued:1.The attributes which have single

value are single valued attributes such roll number.

2.The attributes which has a set of values are called multivalued attributes. Such as any student can have more than 1 telephone number .

TYPES OF ATTRIBUTES

Derived attribute:

•These attributes are derived from the existing attributes of entities.

•The value of a derived attribute is not stored , but computed when required.

Student

Name Address Class Roll number Date-of-birth

TYPES OF ATTRIBUTES

Descriptive attribute:

•The attribute of the relationship is called descriptive attribute.

Cardinality

One to manyMany to oneOne to oneMany to many

It expresses the number of entities to which It expresses the number of entities to which another entity can be associated via a another entity can be associated via a relationship set.relationship set.

ONE TO MANY

A B

a1

a4

a2

a3

b4

b3

b2

b1

MANY TO ONE

b2

a1

a2

a5b3

b2

b1

a3

a4

A B

ONE TO ONE

a1

a2

a3

a4 b4

b3

b2

b1

A B

MANY TO MANY

a1

a3

a2

b4

b3

b2

b1

a4

A B

ROLE

Not specified when entity sets that participate in a relationship set are distinct.

Clarification is needed only when the entity sets of a relationship set are not distinct.

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

RECURSIVE RELATIONSHIP

When entities are self-linked then When entities are self-linked then such a relationship is called a such a relationship is called a recursive relationship.recursive relationship.

TOTAL PARTICIPATION

The participation of an entity set E in a The participation of an entity set E in a relationship set R is said to be TOTAL if relationship set R is said to be TOTAL if every entity in E participates in at least one every entity in E participates in at least one relationship in R.relationship in R.

Student A

Student C

Student B

Mr. C

Mr. B

Mr. A

ENTITY SET : STUDENT ENTITY SET : TEACHER

RELATIONSHIP SET : SUBJECT

PARTIAL PARTICIPATION

The participation an entity set E in a The participation an entity set E in a relationship set R is said to be PARTIAL if relationship set R is said to be PARTIAL if only some entities in E participate in only some entities in E participate in relationships in R.relationships in R.

MR.A

MR.C

MR.D

MR.B

CALCULUS

JAVA

DBMS

FACULTY COURSESRELATIONSHIP SET:COURSE

Design issues

Example : should location be an entity or attribute?

Can a project be in more than one location?

Does location need to be described in more detail?

Weak entity set

Existence of weak entity set depends on the existence of a strong entity set which is called the identifying entity set.

The relationship is always many-to-one from weak to identifying.

Participation of the weak entity set in the relationship must be total.

An entity set that does not have sufficient attributes to An entity set that does not have sufficient attributes to form a primary key is referred to as a weak entity set.form a primary key is referred to as a weak entity set.

Part-2

Introducing to Identifiers

Identifiers Candidate Key

• All available unique, required identifiers of an entity

Primary Key• A unique, required identifier of an entity

• The best one of candidate keys

Foreign Key• A reference key to another entity

• This key is the primary key of another entity

Primary Key

Natural primary key:A natural primary key is one that exists in

the data itself. Surrogate primary key:

The primary key that you have created where non existed before.

Primary key requires the consideration not just of the current data, but of possible future data as well.

Why is it important to identify primary key?

The primary key is the main “Handle” that the database server uses to grab the information in which you are interested.

By identifying a primary key, you are telling the server which information you want to work with at the moment.

Foreign Key

A primary key of one table that is included in another table.

Links the records in one database table to another table.

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