Lecture Database management systems Chapter 2 Entity relationship model

Chapter 2

Entity Relationship
Model


Introduction
Modeling databases using a graphical
technique is proposed so that humans can
relate to easily.
 ER Model is based on a perception of a
real world that consists of collection of
basic objects called entities and
relationships among these objects.



Overview of Database Design


Conceptual design: (ER Model is used at this
stage.)



A database “schema” in the ER Model can be
represented pictorially (ER diagrams).



The next step in DB design is the actual

implementation of the database, using a
commercial DBMS



By using the DBMS, conceptual design from
high-level data model will be transformed into the
implementation data model.


COMPANY example


The COMPANY database keeps track of
company’s employee, departments, and
projects



Suppose that after the requirement
collection and analysis phase, the database
designers provide the following
description:


COMPANY example


The company is organized into
DEPARTMENTs.






Each department has a name, number and an employee
who manages the department.
The start date of the department manager will be kept
track. A department may have several locations.

Each department controls a number of
PROJECTs.


Each project has a unique name, unique number and is
located at a single location.


COMPANY example


We store each EMPLOYEE’s name social
security number, address, salary, sex, and
birthdate.
Each employee works for one department but may
work on several projects.
 We keep track of the number of hours per week that an
employee currently works on each project. We also
keep track of the direct supervisor of each employee.




Each employee may have a number of
DEPENDENTs.


For each dependent, we keep track of their name, sex,
birthdate, and relationship to the employee.


ER Model Basics


ER model describes data
relationships, and attributes

as

entities,



Entity:
an object that exists and is
distinguishable from other objects. In other
words, the entity can be uniquely identified.
◦ Ex:
 A particular person, for example Dr. Alain is an entity.
 A particular department, for example CSE faculty.

 A particular place, for example HCM city can be an
entity.


ER Model Basics


Entity type or entity set: collection of
similar entities
◦ Ex:
 All students in CSE, say STUDENT.
 All courses in CSE, say COURSE



An entity may belong to more than one
entity type.
◦ Ex: The same person is a LECTURER at one
instance and STUDENT at another instance.


ER Model Basics
Relationship: associations among two or
more entities
 Ex:


◦ An Dang works in Pharmacy department.
◦ Teaches is the relationship type between
LECTURER and STUDENT.

Symbols Used in ER Diagram
Entity sets
Attributes
Relationships


ER Model Basics
Attributes: properties of entity
 Ex: Roll number, name, and grade are the
attributes of STUDENT.



Attribute classification

1


Attribute classification


Single Value Attribute: only one value
associated with that attribute.
◦ Symbol for Single Value Attribute:

◦ Ex
STUDENT

FullName


Attribute classification


Multivalued Attribute: more than one
value will be associated with that
attribute.
◦ Symbol:

Skills

◦ Ex:
EMPLOYEE

Name


Attribute classification


Derived Attribute:
◦ Can be computed from other attributes
◦ Symbol:

◦ Ex: Age of a person can be derived from the
date of birth of the person
Age

PERSON
Name


Attribute classification


Composite Attribute


ER Diagram


Key attribute


An important constrain on the entities of
an entity type is the KEY on attributes



An attribute of an entity type for which
each entity must have a unique value is
called a key attribute of the entity type.



For example, SSN of EMPLOYEE.



Each key is underlined


Relationship degree


Unary Relationship (recursive
relationship):
◦ In the unary relationship the number of
associated entity is one. An entity related to
itself is known as recursive relationship.

PLAYER

Captain of


Relationship degree


Binary Relationship: two entities are
involved.
Staff

Is assigned

Department

Relationship degree


Quaternary Relationship: two entities are
involved.
Location

Staff

Is assigned

Department


Relationship degree


Quaternary Relationships: involve 4
entities
SLIDES

PROFESSOR

Teaches

COURSE

STUDENT

Relationship Classification


One-to-Many Relationship Type:
◦ The relationship that associates one entity to
more than one entity is called one-to-many
relationship.
◦ Symbol:
◦ Example: parent–child relationship. For one
parent there can be more than one child.
DEPARTMENT

EMPLOYEE


Relationship Classification


One-to-One Relationship Type:
◦
◦
◦
◦

a special case of one-to-many relationship.
True one-to-one relationship is rare.
Symbol:
Ex: The relationship between the President
and the country is an example of one-to-one
relationship.

PRESIDENT

COUNTRY


Relationship Classification


Many-to-Many Relationship Type
◦ Ex: The relationship between students and
courses.
◦ Symbol: