Lecture Database management systems Chapter 2 Entity relationship model

Lecture Database management systems Chapter 2 Entity relationship model. This chapter includes contents Introduction, overview of database design, ER model basics, symbols used in ER diagram, attribute classification, ER diagram, relationship degree, relationship classification,...

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

 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

◦ 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

AttributesRelationships

ER Model Basics

 Attributes: properties of entity

 Ex: Roll number, name, and grade are the attributes of STUDENT

Attribute classification

1

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

Relationship Classification

 One-to-Many Relationship Type:

◦ The relationship that associates one entity to more than one entity is called one-to-many

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:

Relationship Classification

 Many-to-One Relationship Type

◦ Ex: The relationship between EMPLOYEE and DEPARTMENT is an example of many- to-one relationship There may be many

EMPLOYEES working in one

DEPARTMENT

Relationship Classification

Reducing ER Diagram to Tables

 Mapping Entities:

◦ Regular entities

 Entity name = Name of the relation or table

 Attributes of ER diagram=Column name of the

table

Reducing ER Diagram to Tables

 Mapping Composite Attribute:

CUSTOMER (Customer-ID, Customer name,

Street, City, State, Zip)

Reducing ER Diagram to Tables

 Mapping Multivalued Attribute:

 EMPLOYEE (Employee-ID, Name, Address)

 EMPLOYEE-SKILL (Employee-ID, Skill)

Reducing ER Diagram to Tables

 Mapping one-to-Many Relationship:

 EMPLOYEE (Customer-ID, Name, Address)

 ORDER (Order-ID, Order-Date, Customer-ID)

Reducing ER Diagram to Tables

 Mapping one-to-Many Relationship:

 EMPLOYEE (Customer-ID, Name, Address)

 ORDER (Order-ID, Order-Date, Customer-ID)

Reducing ER Diagram to Tables

 Mapping Many-to-Many Relationship:

 EMPLOYEE(Customer-ID, Name, Other attributes)

 SHIPMENT(Shipment-No, Customer-ID, Vendor-ID, Date Amount)

 ORDER (Vendor-ID, Address, Other Attributes)