Entity Relationship Diagram

E- R DATA MODELING
 An entity is an object that exists and is distinguishable from other
objects.
 Example: specific person, company, event, plant
 Entities have attributes
 Example: people have names and addresses

 An entity set is a set of entities of the same type that share the same
properties.
 Example: set of all persons, companies, trees, holidays

ATTRIBUTES
 An entity is represented by a set of attributes, that is descriptive properties
possessed by all members of an entity set.
 Example:
instructor = (ID, name, street, city, salary )
course= (course_id, title, credits)
 Domain – the set of permitted values for each attribute
 Attribute types:
 Simple and composite attributes.
 Single-valued and multivalued attributes
 Derived attributes

TYPES OF ATTRIBUTES
Simple Attribute: Attribute that consist of a single atomic value.
Example: Salary
Composite Attribute : Attribute value not atomic.
Example : Address : ‘House _no:City:State
Name : ‘First Name: Middle Name: Last Name’

TYPES OF ATTRIBUTES
Single Valued Attribute: Attribute that hold a single value
Exampe1: City
Example2:Customer id
Multi Valued Attribute: Attribute that hold multiple values.
Example1: A customer can have multiple phone numbers, email id's etc
Example2: A person may have several college degrees
Derived Attribute: An attribute that’s value is derived from a stored attribute.
Example : age, and it’s value is derived from the stored attribute Date of Birth.

ENTITY SETS INSTRUCTOR AND STUDENT
instructor_ID instructor_name student-ID student_name

RELATIONSHIP SETS
 A relationship is an association among several entities
Example:
44553 (Peltier) advisor 22222 (Einstein)
student entity relationship set instructor entity

8
ENTITY-RELATIONSHIP DIAGRAMS
 Representing entities
 we represent an entity by a named rectangle
 use a singular noun, or adjective + noun
 refer to one instance in naming
PART-TIME
EMPLOYEE
CUSTOMER

 Representing relationship
DEPT employs EMPLOYEE

Types of Relationships
 Three types of relationships can exist between entities
 One-to-one relationship (1:1): One instance in an entity
(parent) refers to one and only one instance in the
related entity (child).
 One-to-many relationship (1:M): One instance in an
entity (parent) refers to one or more instances in the
related entity (child)

One to one One to many

 Types of Relationships
 Many-to-many relationship (M:N): exists when one instance of
the first entity (parent) can relate to many instances of the
second entity (child), and one instance of the second entity can
relate to many instances of the first entity.
Many to many

CARDINALITY CONSTRAINTS
 We express cardinality constraints by drawing either a directed line
(), signifying “one,” or an undirected line (—), signifying “many,”
between the relationship set and the entity set.
 Or, by numbering each entity. * or, m for many.
 One-to-one relationship:
 A student is associated with at most one instructor via the
relationship advisor
 A student is associated with at most one department via
stud_dept
1 1

ONE-TO-MANY RELATIONSHIP
 one-to-many relationship between an instructor and a student
 an instructor is associated with several (including 0) students via
advisor
 a student is associated with at most one instructor via advisor,
1 *

MANY-TO-MANY RELATIONSHIP
 An instructor is associated with several (possibly 0) students via
advisor
 A student is associated with several (possibly 0) instructors via
advisor
* *

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 Crow’s foot notation: A type of cardinality notation. It is
called crow's foot notation because of the shapes, which
include circles, bars, and symbols, that indicate various
possibilities.
 A single bar indicates one, a double bar indicates one
and only one, a circle indicates zero, and a crow's foot
indicates many.

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Crow's foot notation is a common method of indicating cardinality. The four examples
show how you can use various symbols to describe the relationships between entities.

DIFFERENT TYPES OF KEYS
 A candidate key of an entity set is a minimal super key
 ID is candidate key of instructor
 course_id is candidate key of course

PRIMARY KEY
 A primary key is a candidate key that is most appropriate to be the
main reference key for the table. As its name suggests, it is the primary
key of reference for the table and is used throughout the database to
help establish relationships with other tables.
 The primary key must contain unique values, must never be null
and uniquely identify each record in the table

FOREIGN KEY
 A foreign key is generally a primary key from one table that appears as a
field in another where the first table has a relationship to the second. In
other words, if we had a table A with a primary key X that linked to a table B
where X was a field in B, then X would be a foreign key in B

DIFFERENT TYPES OF KEYS
A super key of an entity set is a set of one or more attributes whose
values uniquely determine each entity.
Example:
 {Student ID,FirstName }
 {Student ID, LastName }
 {Student ID,FirstName,LastName}

E-R DIAGRAMS
 Rectangles represent entity sets.
 Diamonds represent relationship sets.
 Attributes listed inside entity rectangle. Or , as oval shape along with the
rectangle.
 Underline indicates primary key attributes

SUMMARY OF SYMBOLS USED IN E-R NOTATION

TOTAL PARTICIPATION OF ENTITY SET
 E.g., A Class entity cannot exist unless related to a Faculty member
entity

WEAK ENTITY SETS
An entity set that does not have a primary key is referred to as a weak entity set
 We underline the discriminator of a weak entity set with a dashed
line.
 We put the identifying relationship of a weak entity in a double
diamond.
 Primary key for section – (course_id, sec_id, semester, year)

Crows feet notation

 Representing attributes
 Rectangle -- Entity
 Ellipses -- Attribute (underlined attributes are [part of] the primary key)
 Double ellipses -- multi-valued attribute
 Dashed ellipses-- derived attribute, e.g. age is derivable from birthdate and
current date.

 Representing attributes

Extended ER Features: Generalization

EXTENDED ER FEATURES: GENERALIZATION
 A bottom-up design process – combine a number of entity sets
that share the same features into a higher-level entity set.
 Specialization and generalization are simple inversions of each
other; they are represented in an E-R diagram in the same way.
 The terms specialization and generalization are used
interchangeably.

SPECIALIZATION AND GENERALIZATION (CONT.)
 Can have multiple specializations of an entity set based on different
features.
 E.g., permanent_employee vs. temporary_employee, in addition to
instructor vs. secretary
 Each particular employee would be
 a member of one of permanent_employee or temporary_employee,
 and also a member of one of instructor, secretary
 The ISA relationship also referred to as superclass - subclass
relationship

UML
 UML: Unified Modeling Language
 UML has many components to graphically model different aspects of
an entire software system
 UML Class Diagrams correspond to E-R Diagram, but several
differences.

Entity Relationship Diagram

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Entity Relationship Diagram