Entity-Relationship Model and Enhanced Entity-Relation Model.
Entity Relationship Model · Entity Relationship Model Logical representation of data for an...
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Entity Relationship Model
Database System
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Entity Relationship Model
Logical representation of data for an organization.
It views the entire system as a collection of entities related to one
another.
It is used to describe the elements of a system and their relationships.
Provide a graphical notation for representing data models in the form
of a diagram called Entity Relationship Diagram (ERD).
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Entity Relationship Diagram
ERD represents 3 components:
◦ Entities
◦ Attributes
◦ Relationship
Entities: A person, place or thing about which the organization
wishes to maintain data. E.g: Teacher, Student (Person), Country , City
(Place), Toy (thing).
In ER Modeling , entity refers to entity set & not single entity instance.
In other words, in Relational Modeling, Entity refer to table not single
row
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Entity Relationship Diagram
Entities:
For example: Student is a entity type which is a set of all students.
In ERD, Entity is represented by:
Entity
Weak Entity
Composite Entity
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Entity Relationship Diagram
Attributes:
A property or characteristics of an entity type that is of interest to the
organization.
For example: st_name, marks, age are the attributes of Student entity.
In ERD, Attribute is represented by:
Simple Attribute
Multi valued Attributes
Derived Attributed
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Entity Relationship Diagram
Relationships:
Association between entities are called Relationship.
It operate in both directions.
In ERD, Relationships are represented by:
Simple Relationship
Identifying Relationship
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Entity Relationship Diagram
Identifying Relationship:
An identifying relationship means that the child table cannot be
uniquely identified without the parent. For example, you have this
situation in the intersection table used to resolve a many-to-many
relationship where the intersecting table's Primary Key is a
composite of the left and right (parents) table's Primary Keys.
Example...
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Entity Relationship Diagram
Cardinalities:
Number of tuples of one entity that can be associated with each tuple
of an other entity:
Mandatory One
Mandatory Many
Option one
Optional many
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Entity Relationship Diagram
Degree of a Relationship
Degree of a relationship is the number of entities
involved in a relationship.
Special cases are unary, binary, and ternary ,where the
degree is 1, 2, and 3, respectively.
The relationship of degree 2 (Binary) are most commonly
used.
• Example for unary relationship : An employee is a manager
of another employee
• Example for binary relationship : An employee works-for
department.
• Example for ternary relationship : customer purchase item
from a shop keeper
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Degree of a Relationship
The types of relationships with respect to degrees:
1. Unary Relationship
2. Binary Relationship
3. Ternary Relationship
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Unary Relationship
• It is a type of relationship that is established between
instances of same entity type.
• This is also called recursive relationship.
• Example:
Person Lives with
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Binary Relationship
It is a type of relationship that is established between instances of
two different entity type.
o This leads to three possible associations between data items:
o One to One (I:I)
o One to many (I:M)
o Many to Many (M:N)
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Binary Relationship
oOne to One (I:I)
In this case,
“for each instance in first entity class, there is only one instance
in the second entity class and for each instance in second
entity class, there is only one instance in the first entity class”
This relationship means that one chairman runs only one
department, similarly one department is run by one chairman.
Chairman Runs Department
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Binary Relationship
oOne to One (I:I)
Other Examples:
s
This relationship means that one country has only one President, Similarly
one President is of only one country.
This relationship means that one manager manages only one Branch of a
bank, Similarly one branch of a bank is managed by one manager.
.
manages
Country has President
Branch Manager
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Binary Relationship
oOne to Many (I:M)
In this case,
“for each instance in first entity class, there can be many
instances in the second entity class and for each instance in
Second entity class, there is only one instance in the first entity
class”
This relationship means that one state can have many cities
but one city is belonged to one state.
State has City
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Binary Relationship
oOne to Many (I:M)
Other Examples:
s
This relationship means that one University has many Deprtments, But
one department is a part of only one University.
.
University has Department
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Binary Relationship
oMany to Many (M:N)
In this case,
“for each instance in first entity class, there can be many
instances in the second entity class and for each instance in
Second entity class, there can be many instances in the first
Entity class”
This relationship means that one student may study many
courses and one course may be studied by many students.
Student studies Course
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Binary Relationship
oMany to Many (M:N)
This relationship means that one employee can learn many
skills and one skill can be learned by many employees.
Reader reads Book
Employee learn Skill
This relationship means that one reader may read many books
and one book may be read by many readers.
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Ternary Relationship
• Ternary relationship exists among the instances of
three entity types.
This relationship means that one or many consultants with one
or many clients signs on one or many contracts.
Consultant signs Client
Contract
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Ternary Relationship
• Ternary relationship exists among the instances of
three entity types.
This relationship means that one or many anylsts with one
or many programmers works on one or many projects.
Analyst Works
on Programmer
Project