1. Structured Analysis Techniques

2. Entity Relationship Diagrams

3. Data models
Most large software systems make use of a large database of information.
An important part of system modelling is to define the logical form of the data processed by the system.

4. Entity Relationship Diagrams
Entity-Relationship diagrams are a data modeling technique.
E-R modelling uses three basic components: entities, attributes, relationships.
Entities
An entity may be defined as:
An entity is a class of persons, places, objects, events or concepts about which we need to capture and store data.
If a bank stores data about you, then to the bank you are an entity.
If a business stores data relating to a piece of paper called an invoice, then this invoice is an entity.

5. Entity Relationship Diagrams
Attributes
An attribute is a data element associated with an entity. For example, the entity SUPPLIER is likely to have attributes such as supplier_name, supplier_address, supplier_phone_number and so forth.
Relationships
Entities are associated with each other via relationships. A relationship is a named association between two or more entity types.
For example:
the relationship PLAYS-FOR between the entity types PLAYER and TEAM.
The relationship CITIZEN-OF between the entity types PERSON and COUNTRY.

6. Entity Relationship Diagrams
Relationships (cont.)
Degree of Relationship. The degree of a relationship may be:
one-to-one (1:1)
one-to-many (1:N)
many-to-many (N:M)
As an example, consider a company database which might contain the following relationships:
The relationship HEAD-OF between entity types MANAGER
and DEPARTMENT is 1:1. This means that a department has at most one head and that a manager is head of at most one
department.

7. Entity Relationship Diagrams
Relationships (cont.)
Examples:
The relationship SUPERVISES between the entity types MANAGER and EMPLOYEE is 1:N. This assumes that a manager may supervise any number of different employees but a given employee is supervised by at most one manager.
The relationship ASSIGNED-TO between the entity types EMPLOYEE and PROJECT is N:M. Thus an employee may be assigned to many different projects and each project may have many employees assigned to it.

8. Notation
Notation
The graphical representations for E-R diagrams are very simple:
A plain rectangle to represent the entity type
Invoice
A labeled line to represent the relationship

9. Degree of Relationship
There are a number of different graphical conventions for representing the degree of each relationship. The following notation will be used within these notes: A B
1:1 A B
1:n

10. Example
For Example:
COURSE
enrols
STUDENT
ONE course has enrolled on it ONE or MORE students. ONE student is enrolled on ONE course.
ONE supplier is sent ONE or MORE orders.
ONE order is sent to ONE supplier.

11. Example For Example: ONE loan refers to ONE book.
ONE book is referred to on ONE loan.

12. Example College Database
An Example:
An ER model 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
department. A lecturer belongs to only one department. Each department
offers many different courses, each of which is taught by a single lecturer.
A student may enrol for many courses offered by different departments.

13. Example College Database
DEPARTMENT
offers
is_in
head_of
COURSE
LECTURER
teaches_on
enrols
STUDENT
ER Diagram - Small College ( solution )

14. Example Local Education Authority
Another Example:
A local education authority (LEA) requires a system to hold details of the
applicants who apply for courses run by local colleges. Each course is
run at only one college, though more than one course is run at each
college. An applicant may apply for several courses. Details are to be
kept of the standard qualifications that the majority of applicants have.

15. Example Local Education Authority
APPLICANT
applies_for
COURSE
has
is_run_by
QUALIFICATIONS
COLLEGE

16. Many to Many relationship
Decomposing a Many-to-Many (N:M) Relationship
Occasionally a many-to-many relationship will occur, relational databases cannot accommodate this relationship type, as such they must be removed.
The situation is easily resolved because it is always possible to replace a many-to-many relationship type with two one-to-many types, by introducing an extra entity type into the diagram.
This 'new' entity type may simply be the invention of the systems analyst, invented for the sole purpose of eliminating the many-to-many relationship type. In other cases, some actual, physical entity might exist which will provide the needed linkage.

17. Many to Many relationship
Imagine a business which buys in a number of parts from its suppliers. Suppose investigation and analysis has shown that:
any one supplier might supply more than one kind of part.
any one kind of part might be bought in from a number of different suppliers.
supplies
SUPPLIER
PART

18. Many to Many relationship
The many-to-many relationship type is removed as follows:
Thinking of an appropriate name for 'entity X' can be difficult. In such cases it is acceptable to form it from the names of the original two entities.
Naming the two new relationships formed by the decomposition can pose a problem however, it is acceptable to leave the relationships blank providing this does not impair the clarity of the diagram.
SUPPLIER
PART X

19. Many to Many relationship
Remember this?
DEPARTMENT
is_in
offers
head_of
COURSE
LECTURER
teaches_on
enrols
STUDENT

20. Many to Many relationship
The n:m relationship is now removed.
DEPARTMENT
is_in
offers
head_of
teaches_on
COURSE
LECTURER
receives
ENROLMENT
STUDENT
performs

21. Guidelines Guidelines for Drawing an Entity-Relationship Diagram:
Select likely entities.
Select an identifier for each of the entities.
Identify relationships between the selected entities.
Sketch an E-R diagram, adding the degree of each relationship.
Decompose any N:M relationships, allocating an identifier to any new entities formed.