CM2020: Introduction to Database Systems

Conceptual Modelling with The Entity Relationship Model

Database Systems 4th edition

Connolly and Begg Chapter 11

Nirmalie Wiratunga

Aim of Lecture

• Outline the steps involved in designing a database• Explain the first phase: Conceptual Modelling• Study a particular conceptual model:

– Entity Relationship (ER) Model

• By the end you should be able to– Explain what conceptual design is, and how it is used– Represent a real-world situation as an ER Model– Understand an ER model constructed by someone else

Role of Conceptual Modelling within the design process

Real-World Organisation /Problem

Logical Data Model

Conceptual Data Model

Physical Model (via DBMS)

Identify key concepts and data needsCreate a conceptual model

Convert model to structures required by database (relational, object-oriented, etc.)

Implement using a DBMS (MSA, ORACLE):create tables, add data, constraints, etc.

– Use human understandable terms, not computer termsTables, Combo Boxes, Foreign Keys…

The Conceptual Data Model

– Implementation independent– One form is the Entity Relationship (ER) model– Basis for the next step: the logical model

• Abstract view of situation– Identify important entities and relationships between them

Library: Books, Members, Carpets

Member borrows Book

Simplicity of ER modelling

Staff

StaffIdNameTitle

WorksOn

StaffIdProjectId

Project

ProjectIdNameStartDate

11∞

∞

ER diagram

Tables

Suppose we wish to record details of staff working on project.

ER diagrams are simpler than the table representation

Works_onProjectStaff

Purpose of ER modelling?

• ER model is simpler and easier to understand– Helps discussions with customers and fellow-workers

• Separates – Modelling the real-world problem situation– Designing the DB tables for a DBMS (e.g. MSA)

• Most large organisations will require ER Modelling

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Components of an ER Model• The ER Model consists of four items:

1. An ER Diagram (using Bachman Notation)- graphical representation of the entities and the

relationships between them2. A formal description of each entity in terms of its

attributes and identifiers3. Descriptions of the meaning of relationships4. Descriptions of any constraints on the system

and of any assumptions made

Note: the completion of an ER Model is iterative and unlikely to be successful first time round

MakesDriver Trip Vehicle

Incurs

Includes

Uses

Food Processor

Expense

Maintenance

Engineer QualificationHolds

CustomerStop

DeliversReceives

Food Processor Type

HasUndergoes

VisitsDate-obtained

Performs

Staff

Example of a realistic ER diagram

How do you obtain an ER Model ?

• Given a specification, you need to identify the following concepts:– Entities: ‘things’ with physical or conceptual

existence - usually nouns– Relationships: between entities - usually verbs– Attributes: of each entities and sometimes

relationships– any constraints or assumptions

Identify ER Concepts in a Specification

Departments control many projects and each department has many employees. Each employee works on only one project at a time. A project’s start datestart date must be before the project’s target completion datecompletion date.

Entities: departments, projects, employees

Relationships: control - between departments and projectshas - between departments and employeesworks_on - between employees and projects

Attributes: Start date, completion date for project. NI number, name, address for employee.

Constraints: A project’s start date must be before its target completion date.

Entity Attribute Attribute RelationshipRelationship

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Entity• Definition

– real-world object distinguishable from other objects and described using a common set of attributes.

– Diagrammatically shown as

• Entity Set: a collection of similar entities– E.g. all employers, all students– the set tends to have the same number of attributes– must have a primary key

entity-name

Entity Examples

• Delivery databaseDrivers Customers Deliveries Invoice

• College registration databaseStudent Instructor Classroom Course

• Restaurant databaseMenu Recipe Ingredient Order

• Personal Music database ?

Descriptions of entities

• Properties of entities are called attributes– Simple or composite e.g. name (firstname, surname)– Identifiers: One or more unique attributes are chosen as an

identifier for the entity (a.k.a. primary key)– Single or multi-valued e.g. hobbies {cycling,reading,music}– Derived e.g. TotalCost (UnitCost * Quantity)

• Entity description = Entity name + identifier + other attributes

• Entity Set: the set of entity descriptions

Entity Sets

Supervisor Student

Supervisor(staffID, name, jobTitle, address)

Student(studentID, name, address, staffId*)

Entity Setsdo NOT include foreign keys

at the conceptual modelling

stageOther Examples of Entity Sets:

Driver(driver#, first-name, surname, address, #points)Exam(module#, student#, grade)

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Relationships• An association among entities.

– Supervisor supervises student

• Relationships are described in terms of their– Functionality – Membership Class– Attributes

Supervisor Studentsupervises

Relationship: FunctionalityEach supervisor can supervise many students, but each student has only one supervisor

Supervisor Studentsupervises

Functionality answers two questions:• Can the supervisor have more than one student? Yes• Can the student have more than one supervisor? No

So we’re interested in the MAX number of each entity involved: is it 1 or many?

Functionality Types

1- to -1 1- to many many to -1 many to many

Functionality types are based on different mappings between entities

Relationship: Membership ClassA supervisor does not have to supervise any students.A student has to have a supervisor.

The membership class answers two questions:• Must the supervisor have at least one student? No• Must the student have at least one supervisor? Yes

So we’re interested in the MIN number of each entity involved: is it 0 or 1?

Where, 0 means optional and 1 means mandatory

Supervisor Studentsupervises

Supervisor’s participation in

supervision is optionalStudent’s participation in supervision

is mandatory

A supervisor may supervise no students, or may supervise many

students

A student must have at least one supervisor, and not more than one supervisor

Combining Functionality and Membership

Supervisor Studentsupervises

Relationship: Example 1

Castle School

High School

Hill School

Hamilton

Stevens

Walters

TeacherSchoolemploy

Brian

Paul

Every school must employ atleast one or more TeachersEvery teacher must work in at most one school

Castle School

High School

Hill School

Hamilton

Stevens

Walters

TeacherSchoolemploy

Brian

Paul

Every school must employ one or more TeachersEvery teacher may work in zero or one school

- Paul does not work in any school

Relationship: Example 2

Relationship: Example 3

Castle School

High School

Hill School

Hamilton

Stevens

Walters

TeacherSchoolemploy

Brian

Paul

Schools can employ many teachers, but some schools have yet to recruit.Some teachers do not work whilst others work in a school.

Relationship: Example 4

Castle School

High School

Hill School

Hamilton

Stevens

Walters

TeacherSchoolemploy

Brian

Paul

Schools can employ many teachers, but some schools have yet to recruit All teachers must work in a school.

Description of Relationships

Supervisor Studentsupervises

Functionality: one to many, written [1:M]many to many, written [M:N]one to one, written [1:1]

Membership class: optional to mandatory, written [o:m]

Description of relationship is therefore:Supervises: Supervisor supervises student [1:M] [o:m]

Description of Relationships with Attributes

works_inAssistant Lab

hours

Assistant works in Lab for many hours per week.

works_in(hours): assistant works in lab [M:N] [o:m]

Description of relationship

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Descriptions of Constraints / Assumptions

• Examples of constraints:– #points on driver’s license must be less than 11– pickup_date must be before delivery_date– Driver title must be Mr, Mrs or Ms

• Examples of Assumptions– Typically involve assumptions made about a

relationship’s membership and/or functionality

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Example ER model: a staff project management system

Supervisor Studentsupervises

Supervisor(StaffID, Name, JobTitle, Address)

Student(StudentID, Name, Address, date_enrolled, completion_date, DOB)

Entity Sets

Supervises: Supervisor supervises student [1:M] [o:m]Relationships

Constraints/AssumptionsStudent’s date_enrolled must be before completion_date

ER diagram

Example ER model: A library System

Entities– Book(ISBN, title, author,…)– Member(MemberID, name, address, phone#, …)

Relationships– Borrows(return-date) – members borrow books [1:M][o:o]

Constraints and assumptions:– A member can borrow up to 6 books at once– Each book can be borrowed by at most one member

ER diagramreturn-date

borrowsMember Book

Contents• Components of an ER model • Entity

– Entity sets and descriptions• Relationship

– Functional and membership class descriptions• Constraints and Assumptions• Putting it all together with example ER Models• Advance concepts

– Inheritance – Recursion– Ternary relationships

Advanced Concepts of ER diagrams

ER Diagram, which is part of the ER Model, can become more complex i.e., we may have the following:– Entity Subsets and Supersets– Complex Relationships

• Involuted or Recursive Relationships• Ternary Relationships

Entity subsets and supersets

makesDriver Trip

MaintenanceEngineerperforms

Staff

Branch has many Staff.

Engineers perform many maintenance tasks.

Drivers make many trips.

Branch has

Unusual Relationships

• Recursive

Employee

manages

• Ternary

Programmer Project

Site

works on

Convert works on into an entity

Summary ER Modelling

– A conceptual design technique

– Is independent of the type of logical model / database you’re going to transform it into

– Contains entities, attributes, relationships and constraints/assumptions

– ER Diagram is a graphical representation using the Bachman notation