Fundamentals of database systems chapter 3

CHAPTER 3
Data Modeling Using the
Entity-Relationship (ER) Model
Slide 1- 2

Copyright © 2016 Ramez Elmasr and Shamkant B. Navathei Slide 3- 3
Chapter Outline
 Overview of Database Design Process
 Example Database Application (COMPANY)
 ER Model Concepts
 Entities and Attributes
 Entity Types, Value Sets, and Key Attributes
 Relationships and Relationship Types
 Weak Entity Types
 Roles and Attributes in Relationship Types
 ER Diagrams - Notation
 ER Diagram for COMPANY Schema
 Alternative Notations – UML class diagrams, others
 Relationships of Higher Degree

Overview of Database Design Process
 Two main activities:
 Database design
 Applications design
 Focus in this chapter on conceptual database
design
 To design the conceptual schema for a database
application
 Applications design focuses on the programs and
interfaces that access the database
 Generally considered part of software engineering

Overview of Database Design Process

Example COMPANY Database
 We need to create a database schema design
based on the following (simplified) requirements
of the COMPANY Database:
 The company is organized into DEPARTMENTs.
Each department has a name, number and an
employee who manages the department. We keep
track of the start date of the department manager.
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.

Example COMPANY Database
(Continued)
 The database will store each EMPLOYEE’s social
security number, address, salary, sex, and birthdate.

Each employee works for one department but may work
on several projects.

The DB will keep track of the number of hours per week
that an employee currently works on each project.

It is required to keep track of the direct supervisor of
each employee.
 Each employee may have a number of
DEPENDENTs.

For each dependent, the DB keeps a record of name,
sex, birthdate, and relationship to the employee.

ER Model Concepts
 Entities and Attributes

Entity is a basic concept for the ER model. Entities are
specific things or objects in the mini-world that are
represented in the database.

For example the EMPLOYEE John Smith, the Research
DEPARTMENT, the ProductX PROJECT

Attributes are properties used to describe an entity.

For example an EMPLOYEE entity may have the attributes
Name, SSN, Address, Sex, BirthDate

A specific entity will have a value for each of its attributes.

For example a specific employee entity may have
Name='John Smith', SSN='123456789', Address ='731,
Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-55‘
 Each attribute has a value set (or data type) associated with
it – e.g. integer, string, date, enumerated type, …

Types of Attributes (1)
 Simple

Each entity has a single atomic value for the attribute. For
example, SSN or Sex.
 Composite
 The attribute may be composed of several components. For
example:

Address(Apt#, House#, Street, City, State, ZipCode, Country), or

Name(FirstName, MiddleName, LastName).

Composition may form a hierarchy where some components
are themselves composite.
 Multi-valued

An entity may have multiple values for that attribute. For
example, Color of a CAR or PreviousDegrees of a STUDENT.

Denoted as {Color} or {PreviousDegrees}.

Example of a composite attribute

Entity Types and Key Attributes (2)
 A key attribute may be composite.

VehicleTagNumber is a key of the CAR entity type
with components (Number, State).
 An entity type may have more than one key.

The CAR entity type may have two keys:

VehicleIdentificationNumber (popularly called VIN)

VehicleTagNumber (Number, State), aka license plate
number.
 Each key is underlined (Note: this is different from the
relational schema where only one “primary key is
underlined).

Entity Set
 Each entity type will have a collection of entities stored in
the database
 Called the entity set or sometimes entity collection
 Previous slide shows three CAR entity instances in the
entity set for CAR
 Same name (CAR) used to refer to both the entity type and
the entity set
 However, entity type and entity set may be given different
names
 Entity set is the current state of the entities of that type that
are stored in the database

Value Sets (Domains) of Attributes
 Each simple attribute is associated with a value
set
 E.g., Lastname has a value which is a character
string of upto 15 characters, say
 Date has a value consisting of MM-DD-YYYY
where each letter is an integer
 A value set specifies the set of values associated
with an attribute
Slide 3- 13

Displaying an Entity type
 In ER diagrams, an entity type is displayed in a
rectangular box
 Attributes are displayed in ovals
 Each attribute is connected to its entity type
 Components of a composite attribute are connected
to the oval representing the composite attribute
 Each key attribute is underlined
 Multivalued attributes displayed in double ovals
 See the full ER notation in advance on the next
slide

NOTATION for ER diagrams

Entity Type CAR with two keys and a
corresponding Entity Set

Initial Conceptual Design of Entity Types
for the COMPANY Database Schema
 Based on the requirements, we can identify four
initial entity types in the COMPANY database:

DEPARTMENT

PROJECT

EMPLOYEE

DEPENDENT
 Their initial conceptual design is shown on the
following slide
 The initial attributes shown are derived from the
requirements description

Initial Design of Entity Types:
EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT

Refining the initial design by introducing
relationships
 The initial design is typically not complete
 Some aspects in the requirements will be
represented as relationships
 ER model has three main concepts:
 Entities (and their entity types and entity sets)
 Attributes (simple, composite, multivalued)
 Relationships (and their relationship types and
relationship sets)
 We introduce relationship concepts next

Relationships and Relationship Types (1)
 A relationship relates two or more distinct entities with a
specific meaning.

For example, EMPLOYEE John Smith works on the ProductX
PROJECT, or EMPLOYEE Franklin Wong manages the
Research DEPARTMENT.
 Relationships of the same type are grouped or typed into
a relationship type.

For example, the WORKS_ON relationship type in which
EMPLOYEEs and PROJECTs participate, or the MANAGES
relationship type in which EMPLOYEEs and DEPARTMENTs
participate.
 The degree of a relationship type is the number of
participating entity types.
 Both MANAGES and WORKS_ON are binary relationships.

Relationship instances of the WORKS_FOR N:1
relationship between EMPLOYEE and DEPARTMENT

Relationship instances of the M:N WORKS_ON
relationship between EMPLOYEE and PROJECT

Relationship type vs. relationship set (1)
 Relationship Type:
 Is the schema description of a relationship
 Identifies the relationship name and the
participating entity types
 Also identifies certain relationship constraints
 Relationship Set:
 The current set of relationship instances
represented in the database
 The current state of a relationship type

Relationship type vs. relationship set (2)
 Previous figures displayed the relationship sets
 Each instance in the set relates individual participating
entities – one from each participating entity type
 In ER diagrams, we represent the relationship type as follows:

Diamond-shaped box is used to display a relationship type

Connected to the participating entity types via straight lines

Note that the relationship type is not shown with an arrow.
The name should be typically be readable from left to right
and top to bottom.

Refining the COMPANY database
schema by introducing relationships
 By examining the requirements, six relationship types are
identified
 All are binary relationships( degree 2)
 Listed below with their participating entity types:
 WORKS_FOR (between EMPLOYEE, DEPARTMENT)
 MANAGES (also between EMPLOYEE, DEPARTMENT)
 CONTROLS (between DEPARTMENT, PROJECT)
 WORKS_ON (between EMPLOYEE, PROJECT)
 SUPERVISION (between EMPLOYEE (as subordinate),
EMPLOYEE (as supervisor))
 DEPENDENTS_OF (between EMPLOYEE, DEPENDENT)

ER DIAGRAM – Relationship Types are:
WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF

Discussion on Relationship Types
 In the refined design, some attributes from the initial entity
types are refined into relationships:
 Manager of DEPARTMENT -> MANAGES
 Works_on of EMPLOYEE -> WORKS_ON
 Department of EMPLOYEE -> WORKS_FOR
 etc
 In general, more than one relationship type can exist
between the same participating entity types
 MANAGES and WORKS_FOR are distinct relationship
types between EMPLOYEE and DEPARTMENT
 Different meanings and different relationship instances.

Constraints on Relationships
 Constraints on Relationship Types
 (Also known as ratio constraints)
 Cardinality Ratio (specifies maximum participation)

One-to-one (1:1)

One-to-many (1:N) or Many-to-one (N:1)

Many-to-many (M:N)
 Existence Dependency Constraint (specifies minimum
participation) (also called participation constraint)

zero (optional participation, not existence-dependent)

one or more (mandatory participation, existence-dependent)

Many-to-one (N:1) Relationship

Many-to-many (M:N) Relationship

Displaying a recursive
relationship
 In a recursive relationship type.
 Both participations are same entity type in
different roles.

For example, SUPERVISION relationships
between EMPLOYEE (in role of supervisor or
boss) and (another) EMPLOYEE (in role of
subordinate or worker).
 In following figure, first role participation labeled
with 1 and second role participation labeled with
2.
 In ER diagram, need to display role names to
distinguish participations.

A Recursive Relationship Supervision`

Recursive Relationship Type is: SUPERVISION
(participation role names are shown)

Weak Entity Types
 An entity that does not have a key attribute and that is identification-
dependent on another entity type.
 A weak entity must participate in an identifying relationship type with an
owner or identifying entity type
 Entities are identified by the combination of:
 A partial key of the weak entity type
 The particular entity they are related to in the identifying relationship
type
 Example:
 A DEPENDENT entity is identified by the dependent’s first name, and
the specific EMPLOYEE with whom the dependent is related
 Name of DEPENDENT is the partial key
 DEPENDENT is a weak entity type
 EMPLOYEE is its identifying entity type via the identifying relationship
type DEPENDENT_OF

Summary of notation for ER diagrams

Relationships of Higher Degree
 Relationship types of degree 2 are called binary
 Relationship types of degree 3 are called ternary
and of degree n are called n-ary
 In general, an n-ary relationship is not equivalent
to n binary relationships
 Constraints are harder to specify for higher-
degree relationships (n > 2) than for binary
relationships

Fundamentals of database systems chapter 3

More Related Content

What's hot

Similar to Fundamentals of database systems chapter 3

More from Bicycle Thief

Recently uploaded

Fundamentals of database systems chapter 3