1. Entity Relationship Diagrams
1. Basic Elements and Rules

2. Software Analysis & Design
ER Model contains detailed descriptions of:
What are the entities and relationships in the enterprise?
What information about these entities and relationships should we store in the database?
What are the integrity constraints or business rules that hold?

3. Data modelling(ERD) vs Process modelling(DFD)
Process modelling (i.e. DFD) shows data stores, how, where, when data are used or changed in an IS
Data modelling (i.e ER) shows definition, structure, and relationship within the data

4. Why data model is most important part of the statement of SW requirement?
Characteristics of data captured during data modelling are crucial in design of DB, program, computer screen, and reports
Data rather than processes are the most complex aspects of many modern IS so require a central role in structuring system requirement
The characteristics of data (length, format, relationship) are reasonably permanen. The paths of data flow are quite dynamic.
Structural information about data is essensial for automatic generation of programs

5. Conceptual Data Modeling and the E-R Diagram
Goal
Capture as much of the meaning of the data as possible
A better design that is scalable and easier to maintain

6. Introduction to Entity-Relationship (E-R) Modeling
Notation uses three main constructs
Data entities
Attributes
Relationships
Entity-Relationship (E-R) Diagram
A detailed, logical representation of the entities, associations and data elements for an organization or business
10.6

7. Entity-Relationship (E-R) Modeling Key Terms
A person, place, object, event or concept in the user environment about which the organization wishes to maintain data
Represented by a rectangle in E-R diagrams
Attribute
A named property or characteristic of an entity that is of interest to an organization
10.7

8. Entity-Relationship (E-R) Modeling Key Terms
Candidate keys and identifiers
Attribute (or combination of attributes) that uniquely identifies each instance of an entity type

9. Notation Guide ENTITY WEAK ENTITY TYPE RELATIONSHIP TYPE
IDENTIFYING RELATIONSHIP TYPE

10. … Notation Guide . . . ATTRIBUTE KEY ATTRIBUTE MULTIVALUED ATTRIBUTE
DERIVED ATTRIBUTE
COMPOSITE ATTRIBUTE
_____
. . .

11. … Notation Guide
1:N FOR E1:E2 IN R 1 N E1 R E2

12. ER Diagram Basics Entity Relationship Attributes Product Keeps Store
descrip
qty
price
pname
manager
Locations
sname
Relationship
Attributes

13. Entity Real-world object distinguishable from other objects
(e.g a student, car, job, subject, building ...)
An entity is described using a set of attributes
The same entity may have different prominence in different scenario
– In the Company database, an employee’s car is of
lesser importance
– In the Department of Transportation’s registration
database, cars may be the most important concept
– In both cases, cars will be represented as entities;
but with different levels of detail

14. Attributes Key Attributes Value Sets of Attributes
Null Valued Attributes
Attribute Types
– Composite Vs. Simple Attributes
– Single-valued Vs. Multi-valued Attributes
– Derived Vs. Stored Attributes
Notation

15. Key Attributes
Key (or uniqueness) constraints are applied to entity types
Key attribute’s values are distinct for each individual entity.
A key attribute has its name underlined inside the oval
Multiple keys are possible
SSN
EMPLOYEE

16. Null Valued Attributes
A particular entity may not have an applicable value for an attribute
Type of Null Values
– Not Applicable
– Unknown
– Missing

17. Composite Vs. Simple Attributes
Composite attributes can be divided into smaller parts which represent simple attributes with independent meaning
Simple Attribute: Aircraft-Type
Complex Attribute: Aircraft-Location which is comprised of :
Aircraft-Latitude
Aircraft-Longitude
Aircraft-Altitude
Notation

18. Single Vs. Multivalued Attributes
Simple attributes can either be single-valued
or multi-valued
Single-valued: Gender = F
Notation
Multi-valued: Degree = {BSc, MTech}

19. Derived Vs. Stored Attributes
Some attribute values can be derived from
related attribute values:
Age ® Date - B-day
Y-Sal ® 12 * M-Sal
Notation
Age
M-sal
B-days
Y-sal
EMPLOYEE

20. Derived Vs. Stored Attributes
Order
Item
price
qty
Total-Value
Some attribute values can be derived from attributed values of related entities
total-value = sum (qty * price)

21. Representing Attributes
Parenthesis ( ) for composite attributes
Brackets { } for multi-valued attributes
Assume a person can have more than one residence and each residence can have multiple telephones
{AddressPhone
({ Phone ( AreaCode,PhoneNum ) },
Address (StreetAddresss (Number, Street, AptNo), City,State,PostalCode) ) }

22. Entity-Relationship (E-R) Modeling Key Terms
An association between the instances of one or more entities that is of interest to the organization
Relationships are always labeled with verb phrases

23. Naming and Defining Relationships
Relationship name is a verb phrase
Avoid vague names
Guidelines for defining relationships
Definition explains what action is being taken and why it is important
Give examples to clarify the action
Optional participation should be explained
Explain reasons for any explicit maximum cardinality

24. Relationship Degree
Departments
The degree of a relationship type is the number of participating entity types
– 2 entities: Binary Relationship
3 entities: Ternary Relationship
– Same entity type could participate in
multiple relationship types
Works_In
Binary
Employees
Multiple
Supply
Assigned_to
Supplier
Project
Ternary
Part

25. Entity Roles Each entity type that participates in a relationship
Employees
Works_In
Departments
employer
worker
Role
Names
Each entity type that
participates in a relationship
type plays a particular role
in the relationship type
The role name signifies the
role that a participating
entity from the entity type
plays in each relationship
instance, i.e. it explains what
the relationship means

26. Relationship Constraints
What are Relationship Constraints ?
Constraints on the relationship type limit the possible combination of entities that may participate in the corresponding relationship set

27. Possible Relations . . . . 1–to-1 (1 : 1) – Both entities can
participate in only one
relationship instance
1-to-Many, Many-to-1
(1 : N, N : 1)
– One entity can
participate in many
relationship instances
Many-to-Many (N: M)
– Both entities can participate in
many relationship instance .
Many - to - 1 . .
1- to - Many .
Many-to-Many

28. Example Cardinality Constraints
How many Employees can work in a Department?
One employee can work in only one department
How many Employees can be employed by a Department?
One department can employ many employees
How many managers can a department have?
One department can have only one manager
How many departments can an employee manage?
One employee can have manage only one department

29. Representing Relations1
Employees
Works_In
Departments
Manages
One employee can work in only one department
One department can employ many employees
One department can have only one manager
One employee can manage only one department