Software Engineering, Lectures

1. SE-381
Software Engineering
BEIT-V
Lecture # 19
(Entity Relationship and Attribute
Analysis & Data Modelling)

2. Data Modeling
– Data Modeling, Semantic Modeling, Entity Relationship
Diagrams, Entity Relationship Attribute Modeling, Object Data
Classes etc are various names for the same
technique, commonly known as Data Modeling.
– Different authors have used slightly different notations to
represent Entities, Relationships and Object Data Classes
– Notations used by Chen 1976 (the founder of the technique)
and Baynon-Davies 1989 are followed. Some unlearning and re-
learning may be required to those who have already learnt and
used it in their Data Bases course
– Data Modeling supplements the Process Modeling technique
and helps in identifying the data structures on which the system
is to be built

3. Entity Relationship Modelling
• One of the Oldest techniques used in Database
and Information Systems Design, proposed by
Chen 1976
• Representation of ‘Real World’ in terms of
Entities, Attributes and Relationships
between the Entities
An Entity is a thing which the enterprise recognizes as being
capable of an independent existence and which can be uniquely
identified (Howe, 1983)
An Entity may be an Object, concept or an event having
characteristics of independent existence and unique
identification, and should have data items or attributes
associated with it.
An Entity together with its attributes define an Entity Type of which
there may be many instances. Similarly, a Relationship Type
may have many instances, where occurrences of allied Entities
are participating

4. Relationship
A Relationship is
an association between two or more entities. (Howe, 1983)
The common attribute in two Entities, indicates they have some
relationship.
For N entities, there could be N(N-1)/2 possible relations, as every
Entity being part of the systems will be related to every other
entity so only direct relationships between the Entities are
considered
Parent Child School
GoesSponsors
Parent and Child and Child and School have direct
relationships, but the relationship between Parent and School is
indirect or derived relationship which should not be considered

5. Cardinality or Degree of the Relationship
• It is number of instances of the respective entity type
participating in the relationship.
• It can be one-to-one (1:1), one-to-many (1:M) or many-to-many
(M:N)
• Crow-Foot notation used to represent cardinality
Class Student
CourseStudent
Cheque PaymentBrings
Consists_of
Read

6. Membership Class or Optionality
• Membership is Mandatory if every occurrence of the entity
participates in the relationship
• Membership is Optional if some of the instances of respective
entity do exist but don’t participate in the relationship, or some
occurrences of the entity can exist independently
– Every Employee must be employed within a Department
– A Department may exist without any (full-time) Employee
• For Mandatory Entity we put in ‘a filled dot’ within the Entity, and
for Optional Entity it is put outside on the Relationship
Cheque PaymentBrings
Employee DeprttWorks
Cheque PaymentBrings

7. Assignment Rules
1:1 Relationship
– Membership Mandatory on Both Entity Types
– Membership Mandatory for only one Entity Type
Cheque PaymentBrings
Single table is needed
Cheque (Cheque_no, date, sign, amount, … , Payment_Id, …)
Employee CarUses
Two tables needed, post the identifier of Optional into Mandatory
Employee (Employee_no, Name, Designation, …)
Car (Reg_no, Model, Capacity, …., Employee_no )

8. Assignment Rules
– Membership Optional for both Entity Types
Employee CarUses
Three tables needed, one for each Entity and one for Relationship
Employee (Employee_no, Name, Designation, …)
Car (Reg_No, Model, Capacity, …. )
Uses ( Employee_no, Reg_no, …., Mileage_used_by)

9. Assignment Rules
1:M Relationship
– Membership of many ‘Entity’ is Mandatory,
Ward PatientContains
Two tables needed
Ward (Ward_no, Location, type, …)
Patient ( Patient_Id,, Name, date_of_Admiss, …, Ward_no)
– Membership of many ‘Entity’ is also Optional
Ward PatientContains
Three tables needed
Ward (Ward_no, Location, type, …)
Patient ( Patient_Id,, Name, date_of_Admiss, …)
Contains (Patient_no, Ward_no)

10. Assignment Rules
M:N Relationship
– Will need Three Tables irrespective of membership class
Teacher StudentTutors
Three tables needed
Teacher (Teacher_name, Subject, Qualification …)
Student ( Reg_no,, Name, date_of_Admiss, Semester …)
Tutors (Teacher_name, Reg_no)

11. Drawing of E-R Diagrams
Following steps be followed to draw E-R Diagrams
1. Identify Entities from the problem definition
2. Investigate and record their inter-relationships
3. Draw E-R diagram showing Entities and Relationships
4. Indicate Degree and Membership Class of each Relationship
5. Validate the ERD against the System Requirements
6. Rationalize
7. Revalidate
8. Using Assignment Rules, convert the diagram into set of fully
normalized tables

12. ERD - A Case Study
Design an appropriate information system for the patients’
appointments and operations activities of a large general
hospital. Initial analysis provide the following brief
description of the existing manual system:
• Patients are dealt with by an appointment system
• Patients must make an appointment for a clinic session held at
one of the hospital’s clinics
• Doctors are allocated one or more appointments within a clinic
session, but only one doctor will be present at each
appointment
• Operations are scheduled and allocated to one of a number of
theatre sessions held in the hospital’s operating theatres, each
doctor may perform a number of given operations on patients
1. First identify the set of Entities from the above description

13. Entities of the System
HOSPITAL
CLINIC
CLINIC-SESSION
APPOINTMENT
PATIENT
DOCTOR
OPERATION
OPERATING-THEATRE
THEATRE-SESSION
2. Investigate and record Inter-relationships
How many total relations are possible, Identify the direct Relations
3. Draw E-R diagram showing Entities and Relationships

14. First-Pass ERD

15. Eliminate the Indirect Relationships
4. Indicate and draw the Optionality of the relationships

16. Second-Pass ERD

17. Set of Tables Generated

18. References
1. NIIT (2005); System Analysis and Design for
Software Engineers; Printice-Hall of India, New
Delhi
2. Paul Beynon-Davies (1989); Information
Systems Development, Macmillan, London, UK;
Ch-5 Entity – Relationship Diagrams pp:38-54
3. P. P-S Chen (1976); the Entity-Relationship
Model – Toward a Unified View of Data; ACM
Transactions on Database Systems 1, pp:9-36
4. Any book on Data Bases or Data Modeling