1. Fundamentals of Databases CSU 07203
The Institute of Finance Management
FCIM
BCS & BIT
Year I -2016/2017
Entity Relationship (ER) Model

2. Entity Relationship (ER) Model
Logistics
Instructor: Siphy, A. S(Mr.) Office: Block D, 020 Consultation Time Thursdays 14:00 -15:00 HRS Or By appointment
Entity Relationship (ER) Model

3. Entity Relationship (ER) model
References;
Elmasri/Navathe: Fundamentals of DB system, 4th Ed, Ch. 3-5, )
Rob/Cornel: Database systems 6 ed.
Entity Relationship (ER) Model

4. Data Modeling Using the Entity-Relationship (ER) Model

5. Entity Relationship (ER) Model
In this lecture;
ER models
Entities
Relationships
Attributes
Business rules
ER Diagrams (ERDs)
Entity Relationship (ER) Model

6. Entity Relationship (ER) Model
Objectives
To illustrate how relationships between entities are defined and refined.
To know how relationships are incorporated into the database design process.
To describe how ERD components affect database design and implementation.
Entity Relationship (ER) Model

7. Entity Relationship (ER) Model
Objective..
Describe the Conceptual Database Design
Describe Major components of ER diagram
Compare and evaluate given ER-diagrams.
Evaluate the significance of the ER-model for DB design.
Describe why understanding of organizational data is important.
Argue why unambiguous representation of data in logical terms is needed for implementing a database that will effectively serve the needs of management
Understanding the business rules
Entity Relationship (ER) Model

8. Database Design
Before we look at how to create and use a database we’ll look at how to design one
Need to consider
What tables, keys, and constraints are needed?
What is the database going to be used for?
Conceptual design
Build a model independent of the choice of DBMS hardware platform that is used to implement the database
Logical design
Create the database in a given DBMS
Physical design
How the database is stored in hardware
Entity Relationship (ER) Model

9. What is Conceptual Database Design?
Process of describing the data, relationships between the data and the constraints on the data.
After analysis - Gather all the essential data required and understand how the data are related
The focus is on the data, rather than on the processes.
The output of the conceptual database design is a Conceptual Data Model ( + Data Dictionary)
Entity Relationship (ER) Model

10. Gathering Information for Conceptual Data Modeling
Two perspectives
Top-down
Data model is derived from an intimate understanding of the business.
Bottom-up
Data model is derived by reviewing specifications and business documents.
Entity Relationship (ER) Model

11. Entity-Relationship (ER) Modeling.
1976 proposed by Peter Chen
ER Modelling is
used for conceptual database design
a top-down approach to database design.
The basic concepts of the E-R model include..
Entity types
Relationship types and
Attributes
Entity Relationship (ER) Model

12. Entity Relationship (ER) Model
E-R Modeling (Cont.)
ER Models are often represented as ER diagrams (ERDs)
ERD is widely used in database design
Represents conceptual level of a database system
Describes things and their relationships in high level
Can identify some problems in a design
The ERDs represent three main components
Entities,
Attributes &
Relationships
ERD example
Lecturer ID
Name
Course
Tutors
Student
Studies
Module
Entity Relationship (ER) Model

13. Entities Entities represent objects or things of interest
Physical things like students, lecturers, employees, products
More abstract things like modules, orders, courses, projects
Entity Relationship (ER) Model

14. Entity Relationship (ER) Model
Entity type & Sets
Entities have a general type /class called entity type.
Entity type
a class of entities that have the same set of attributes
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
Entity Relationship (ER) Model

15. Entity Relationship (ER) Model
Entities…
More examples of entities:
Person: EMPLOYEE, STUDENT, PATIENT
Place: STORE, WAREHOUSE
Object: MACHINE, PRODUCT, CAR
Event: SALE,REGISTRATION, RENEWAL
Concept: ACCOUNT, COURSE
Guidelines for naming and defining entity types:
An entity type name is a singular noun
An entity type should be descriptive and specific
and concise
Event entity types should be named for the result of the event, not the activity or process of the event.
Entity Relationship (ER) Model

16. Diagramming Entities - example
In an ER Diagram, an entity is usually drawn as a box with rounded corners/ rectangular shape
The box is labelled with the name of the class of objects represented by that entity
Lecturer ID
Name
Course
Tutors
Student
Studies
Module
Diagramming Entities - example
Entity Relationship (ER) Model

17. Attributes
Attributes are facts, aspects, properties, or details about an entity
Attributes have
A name
An associated entity
Domains of possible values
Values from the domain for each instance of the entity they are belong to
Entity Relationship (ER) Model

18. Entity Relationship (ER) Model
Attributes..
Example of entity types and associated attributes:
STUDENT: Student_ID, Student_Name, Home_Address, Phone_Number, Major
Guidelines for naming attributes:
An attribute name is a noun.
An attribute name should be unique
To make an attribute name unique and clear, each attribute name should follow a standard format
Similar attributes of different entity types should use similar but distinguishing names.
Entity Relationship (ER) Model

19. Diagramming Attributes
In an ER Diagram attributes may be drawn as ovals
Each attribute is linked to its entity by a line
The name of the attribute is written in the oval
Lecturer ID
Name
Course
Tutors
Student
Studies
Module
Entity Relationship (ER) Model

20. Entity Relationship (ER) Model
Attributes -Types
An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.
Classifications of attributes:
Required vs. Optional Attributes
Simple vs. Composite Attribute
Single-Valued vs. Multivalued Attribute
Stored vs. Derived Attributes
Identifier Attributes - keys
Attribute types:
Simple and composite attributes.
Single-valued and multi-valued attributes
E.g. multi-valued attribute: phone-numbers, project, hobby and so on
Derived attributes
Can be computed / derived from other attributes
E.g. age, given date of birth
Entity Relationship (ER) Model

21. Required vs. Optional Attributes
Example: entity – ONLINE_ACCOUNT
Required attributes (Not NULL)
Account_ID
Password
Owner_Name
Optional attributes (NULL)
Phone_Number
Password_Hint
Entity Relationship (ER) Model

22. Entity Relationship (ER) Model
Figure 1 – A composite attribute
Composite: Is an attribute composed of multiple components, each with an independent existence
Composite attribute
An attribute broken into component parts
component attributes
Entity Relationship (ER) Model 12

23. Entity Relationship (ER) Model
Figure 2 – Entity with a Multivalued attribute (Skill) and derived attribute (Years_Employed)
Multivalued: Is one that holds multiple values for a single entity
Derived: is one that represents a value that is derivable from the value of a related attribute or set of attributes , not necessarily in the same entity
Derived
from date employed and current date
Multivalued:
an employee can have
more than one skill
Entity Relationship (ER) Model 13

24. Entity Relationship (ER) Model
Figure 3 – An attribute that is both Multivalued and composite
Entity Relationship (ER) Model 37

25. Entity Relationship (ER) Model
Identifiers (Keys)
Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type
Candidate Key – an attribute that could be a key. Examples (for STUDENT, PERSON)?
Simple Key versus Composite Key
Entity Relationship (ER) Model 6

26. Entity Relationship (ER) Model
Figure 4 Simple key attribute
The key is underlined
Entity Relationship (ER) Model 14

27. Entity Relationship (ER) Model
Figure 5 Composite key attribute
Flight_Number
Date
Entity Relationship (ER) Model 15

28. Relationship name: writes
Relationships
Associations between instances of one or more entity types that is of interest
Given a name that describes its function.
relationship name is an active or a passive verb.
Relationship name: writes
Book
Author
An author writes one or more books A book can be written by one or more authors.
Entity Relationship (ER) Model

29. Entity Relationship (ER) Model
Relationships..
Relationships have
A name
A set of entities that participate in them
A degree - the number of entities that participate (most have degree 2)
A cardinality ratio
Entity Relationship (ER) Model

30. Diagramming Relationships
Relationships are links between two entities
The name is given in a diamond box
The ends of the link show cardinality
Lecturer ID
Name
Course
Tutors
Student
Studies
Module
One
Many
Entity Relationship (ER) Model

31. Cardinality and Connectivity
Relationships can be classified as either
One – to – One (1:1)
One – to – Many(1:M)
Many – to –Many(N:M)
Cardinality : minimum and maximum number of instances of Entity B that can (or must be) associated with each instance of entity A.
Connectivity
Entity Relationship (ER) Model

32. Cardinality Ratios.. One to many (1:M) Many to many (M:M)
Each entity in a relationship can participate in zero, one, or more than one instances of that relationship
Three types of relationships (examples)
One to one (1:1)
Each lecturer has a unique office
One to many (1:M)
A lecturer may tutor many students, but each student has just one tutor
Many to many (M:M)
Each student takes several modules, and each module is taken by several students
Entity Relationship (ER) Model

33. Cardinality and Connectivity… examples
Professor
teaches
Class
Professor
Class
teaches
A professor teaches class OR
A class is taught by professor
How Many??
Entity Relationship (ER) Model

34. Cardinality and Connectivity…examples1 M
Professor
teaches
Class
(1,4)
(1,1)
Cardinality
Connectivity
Professor
Class
teaches
(1,1)
(1,4)
Cardinality
Entity Relationship (ER) Model

35. ER Modeling Notations -summary
Notation uses three main constructs
Data entities
Relationships
Attributes
Chen Model & Crow’s Foot Model
Entity Relationship (ER) Model

36. Entity Relationship (ER) Model
Chen Notation
Association between the instances of one or more entity types
EntityName
Verb Phrase
AttributeName
named property or characteristic of an entity
Person, place, object, event or concept about which data is to be maintained
Represents a set or collection of objects in the real world that share the same properties
Entity set
Entity Relationship (ER) Model

37. Entity Relationship (ER) Model
Crow’s Foot Notation
Entity
Attribute
Relationship
EntityName
EntityName
Verb phrase
List of Attributes
Acceptable
Entity Relationship (ER) Model

38. Entity Relationship (ER) Model
Connectivity
Chen Model
1 to represent one.
M to represent many
Crow’s Foot 1 M
Mandatory one , means (1,1)
One
many
One or many
Entity Relationship (ER) Model

39. Entity Relationship (ER) Model
Relationships….
Relationship Type vs. Relationship Instance
Degree of a relationship
Cardinality of a relationship
Associative Entity – combination of relationship and entity
Entity Relationship (ER) Model

40. Entity Relationship (ER) Model
Relationship Sets
A relationship set is a mathematical relation among
n  2 entities, each taken from entity sets
{(e1, e2, … en) | e1  E1, e2  E2, …, en  En} where (e1, e2, …, en) is a relationship
Example:
(Hayes, A-102)  depositor
Entity Relationship (ER) Model

41. Relationship Set borrower example
Entity Relationship (ER) Model

42. Relationship Sets (Cont.)
Entity Relationship (ER) Model

43. Entity Relationship (ER) Model
Figure 6 –a relationship with an attribute
attribute of the relationship
The relationship type is modeled as the diamond and lines between entity types
Entity Relationship (ER) Model 20

44. Entity Relationship (ER) Model
Degree of a relationship is the number of entity types that participate in it
Entities of two different types related to each other
One entity related to another of the same entity type
Entities of three different types related to each other
Entity Relationship (ER) Model 8

45. Entity Relationship (ER) Model
Degree of relationship -Examples
Refers to number of entity sets that participate in a relationship set. It indicates the number of associated entities
Entity Relationship (ER) Model

46. Cardinality of a Relationship
One-to-One
Each entity in the relationship will have exactly one related entity
One-to-Many
An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity
Many-to-Many
Entities on both sides of the relationship can have many related entities on the other side
Entity Relationship (ER) Model

47. Cardinality of a Relationship-examples
One to One
One to Many
One to One
Note: Some elements in A and B may not be mapped to any
elements in the other set
Entity Relationship (ER) Model

48. Cardinality of a Relationship- examples
Many to one
Many to Many
Note: Some elements in A and B may not be mapped to any
elements in the other set
Entity Relationship (ER) Model

49. Entity Relationship (ER) Model
Figure 10 –Examples of relationships of different degrees –Unary relationships
Entity Relationship (ER) Model 22

50. Entity Relationship (ER) Model
Figure 11 –examples of relationships of different degrees –Binary relationships
Entity Relationship (ER) Model 23

51. Entity Relationship (ER) Model
Figure 12 –examples of relationships of different degrees –Ternary relationships
Note: a relationship can have attributes of its own
Entity Relationship (ER) Model 24

52. Cardinality Constraints
Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity
Minimum Cardinality. Examples?
If zero, then optional
If one or more, then mandatory
Maximum Cardinality. Examples?
The maximum number
Entity Relationship (ER) Model 29

53. Entity Relationship (ER) Model
Cardinalities
Mandatory one
Mandatory many
Optional one
Optional many 11 18
Entity Relationship (ER) Model

54. Entity Relationship (ER) Model
Figure 13a –Basic relationship
Figure 13b –Optional cardinalities with unary degree, one-to-one relationship
Entity Relationship (ER) Model 1

55. Maximum Cardinality Constraint
max cardinality constraint
Entity Relationship (ER) Model 41

56. Entity Relationship (ER) Model
Examples of multiple relationships –employees and departments
Entities can be related to one another in more than one way
Entity Relationship (ER) Model 40

57. Attributes or Relationship?
Using relationships to link related attributes –composite, Multivalued attribute versus relationship
Attributes can be transformed to relationships
Entity Relationship (ER) Model

58. Entity Relationship (ER) Model
Associative Entities
It’s an entity – it has attributes
AND it’s a relationship – it links entities together
When should a relationship with attributes be an associative entity?
All relationships for the associative entity should be many
The associative entity could have meaning independent of the other entities
The associative entity should have at least one or more attributes other than the identifier
The associative entity may participate in other relationships other than the entities of the associated relationship
Ternary relationships should be converted to associative entities
Entity Relationship (ER) Model

59. Removing M:M Relationships
Many to many relationships are difficult to represent
We can split a many to many relationship into two one to many relationships
An entity represents the M:M relationship
Enrolment
Student
Module In
Has
Student
Module
Studies
Entity Relationship (ER) Model

60. Entity Relationship (ER) Model
An associative entity (CERTIFICATE)
Entity Relationship (ER) Model 21

61. Entity Relationship (ER) Model
Bill of materials structure
Entity Relationship (ER) Model 27

62. Entity Relationship (ER) Model
Figure ; Ternary relationship as an associative entity
Entity Relationship (ER) Model 36

63. Strong vs. Weak Entities
Strong entities
exist independently of other types of entities
has its own unique identifier
represented with single-line rectangle
Weak entity
dependent on a strong entity…cannot exist on its own
does not have a unique identifier
represented with double-line rectangle
Identifying relationship
links strong entities to weak entities
represented with double line diamond
Entity Relationship (ER) Model

64. Figure ; Example of a weak entity and its identifying relationship
-ER notation
Strong entity
Identifying relationship
Weak entity

65. Entity Relationship (ER) Model
Basic E-R notation
Relationship symbols
Attribute symbols
Entity symbols
A special entity that is also a relationship
Relationship degrees specify number of entity types involved
Relationship cardinalities specify how many of each entity type is allowed
Entity Relationship (ER) Model 8

66. Entity Relationship (ER) Model
Sample E-R Diagram
Attribute
Supplier_ID
Supplier_Name
Supplier_Address
Entity Relationship (ER) Model 3

67. Entity Relationship (ER) Model
Design Issues
Identify business rules
Construct ERD
Use of entity sets vs. attributes Choice mainly depends on the structure of the enterprise being modeled, and on the semantics associated with the attribute in question.
Use of entity sets vs. relationship sets Possible guideline is to designate a relationship set to describe an action that occurs between entities
Binary versus n-ary relationship sets Although it is possible to replace any non-binary (n-ary, for n > 2) relationship set by a number of distinct binary relationship sets, a n-ary relationship set shows more clearly that several entities participate in a single relationship.
Placement of relationship attributes
Entity Relationship (ER) Model

68. Making ER Models To make an E/R model you need to identify
Entities
Attributes
Relationships
Cardinality ratios
from a description
General guidelines
Since entities are things or objects they are often nouns in the description
Attributes are facts or properties, and so are often nouns also
Verbs often describe relationships between entities
Entity Relationship (ER) Model

69. Entity Relationship (ER) Model
Business Rules
Brief, precise, and unambiguous descriptions of a policies, procedures, or principles within a specific organization
Apply to any organization that stores and uses data to generate information
Description of operations that help to create and enforce actions within that organization’s environment
Entity Relationship (ER) Model

70. Business Rules (continued)
Must be rendered in writing
Must be kept up to date
Sometimes are external to the organization
Must be easy to understand and widely disseminated
Describe characteristics of the data as viewed by the company
Entity Relationship (ER) Model

71. Discovering Business Rules
Sources of Business Rules:
Company managers
Policy makers
Department managers
Written documentation
Procedures
Standards
Operations manuals
Direct interviews with end users
Entity Relationship (ER) Model

72. Discovering Business Rules (continued)
Generally,
Nouns translate into entities
Verbs translate into relationships among entities
Relationships are bi-directional
Entity Relationship (ER) Model

73. Business Rules Examples - ER Diagrams
Draw the ERD for the following scenarios:
Each patient has one or more patient histories; each instance of patient history belongs to one patient.
An employee may be recorded as having many jobs; a particular job may be recorded as having been held by many employees.
A customer may place many orders; an order is specific to one customer.
Entity Relationship (ER) Model

74. Example
A university consists of a number of departments. Each department offers several courses. A number of modules make up each course and a module can be part of many courses. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students
Entity Relationship (ER) Model

75. Example - Entities
A university consists of a number of departments. Each department offers several courses. A number of modules make up each course and a module can be part of many courses. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students
Entity Relationship (ER) Model

76. Example - Relationships
A university consists of a number of departments. Each department offers several courses. A number of modules make up each course and a module can be part of many courses. Students enrol in a particular course and take modules towards the completion of that course. Each module is taught by a lecturer from the appropriate department, and each lecturer tutors a group of students
Entity Relationship (ER) Model

77. Example - E/R Diagram Department Course Module Lecturer Student
Entities: Department, Course, Module, Lecturer, Student
Department
Course
Module
Lecturer
Student
Entity Relationship (ER) Model

78. Example - E/R Diagram Department Course Module Lecturer Student
Each department offers several courses
Offers
Department
Course
Module
Lecturer
Student
Entity Relationship (ER) Model

79. Example - E/R Diagram Department Course Module Lecturer Student
A number of modules make up each courses
Offers
Department
Includes
Course
Module
Lecturer
Student
Entity Relationship (ER) Model

80. Example - E/R Diagram Department Course Module Lecturer Student
Students enrol in a particular course
Offers
Department
Includes
Course
Module
Lecturer
Enrols In
Student
Entity Relationship (ER) Model

81. Example - E/R Diagram Department Course Module Lecturer Student
Students … take modules
Offers
Department
Includes
Course
Module
Lecturer
Takes
Enrols In
Student
Entity Relationship (ER) Model

82. Example - E/R Diagram Department Course Module Lecturer Student
Each module is taught by a lecturer
Offers
Department
Includes
Teaches
Course
Module
Lecturer
Takes
Enrols In
Student
Entity Relationship (ER) Model

83. Example - E/R Diagram Department Course Module Lecturer Student
a lecturer from the appropriate department
Offers
Employs
Department
Includes
Teaches
Course
Module
Lecturer
Takes
Enrols In
Student
Entity Relationship (ER) Model

84. Example - E/R Diagram Department Course Module Lecturer Student
each lecturer tutors a group of students
Offers
Employs
Department
Includes
Teaches
Course
Module
Lecturer
Takes
Enrols In
Tutors
Student
Entity Relationship (ER) Model

85. Entity Relationship (ER) Model
Example - E/R Diagram
Draw an ERD for a small college database comprising the entity types: DEPARTMENT, COURSE, STUDENT and LECTURER.
 Each department has many lecturers, one of whom is the head of department. A lecturer belongs to only one department. Each department offers many different courses, and many lecturers can teach on a single course. Lecturers can also teach on more than one course. A student may enrol for many courses offered by different departments.
Entity Relationship (ER) Model

86. Symbols Used in E-R Notation-summary
The attributes are connected using lines to the entities. If the attribute is simple or single valued a single line is used
If the attributes is derived a dotted line is used
If it is multi-valued then double line are used
If the attribute is composite, its component attributes are shown as ellipse emanating from the composite attribute
Entity Relationship (ER) Model

87. Symbols Used in E-R Notation(cont.)
Entity Relationship (ER) Model

88. Alternative E-R Notations
Entity Relationship (ER) Model