1. Data Modeling Using the ERD
Data Base 1
Data Modeling Using the ERD

2. E-R Model Introduction
There are different methods of data modeling, but the most widely used is the Entity-Relationship (ER) model.
ER-Model can simply be stated as data model of the real world as entities and relationships. A basic component of the model is the Entity-Relationship diagram which is used to visually represent data objects.

3. To make ERD we need to make the following:
Step1: What are entities? (Tables)
Step 2:What attributes includes in each table
Step 3:What the primary key attribute in each table
Step 4: What are relations between tables
Step 5: Show the degree of participation for each table in relation

4. Summary of notation for ER diagrams

5. Representation in ER-Model
Entities
Relationships
Attributes
Connectivity and Cardinality
ER Notation

6. Entities
Entities are the prime data objects about which information is to be collected. Entities are usually recognizable concepts, either concrete or abstract, such as person, places, things, or events which have relevance to the database.
Examples - Employees and Customers

7. Relationships
A Relationship represents an association between two or more entities.
Example - Employees are assigned projects.

8. Attributes
Attributes describe the entity or relationship of which they are associated.
Example - Troy Griffin (employee’s name)

9. Connectivity and Cardinality
The connectivity of a relationship describes the mapping of associated entity instances in the relationship. The cardinality of a relationship is the actual number of related occurrences for each of the two entities. There are three basic types of connectivity:
1. A one-to-one relationship.
2. A one-to-many relationship.
3. A many-to-many relationship.

10. Connectivity and Cardinality
A one-to-one relationship which is when at most one instance of an entity A is associated with one instance of entity B.

11. Connectivity and Cardinality
A one-to-many relationship which is when for one instance of entity A, there are zero, one, or many instances of entity B, but for one instance of entity B, there is only one instance of entity A.

12. Connectivity and Cardinality
A many-to-many relationship which is when for one instance of entity A, there are zero, one, or many instances of entity B and for one instance of entity B there are zero, one, or many instances of entity A.

13. ER Notation
There is no standard for representing data objects in ER diagrams. Each modeling style uses its own notation. All notational styles represent entities as rectangular boxes and relationships as lines connecting boxes. The standard given below is widely used:
Attributes - Ellipses
Relationship Sets - Diamonds
Cardinalities – An arrow or a 1 pointing where one entity can be mapped

14. E-R Constraints

15. What are constraints?
A constraint is assertion about a database that must be true at all times.
It is part of database schema.
Constraints play an important role in determining the best database design for an enterprise.

16. Why are constraints important?
It gives more semantics to the data
Allow to refer to entities eg using keys
Enables efficient storage, lookup etc.

17. Types of constraints
Participation constraint - If all entities in an entity set must participate in a relation in the relationship set, a thick line is drawn.
Two types
Total – all entities of the entity set should participate in a relation in the relationship set. With total participation Eg. every loan has to be associated with a customer
Partial – not all entities of the relation has to participate in a relation in the relationship set. Eg. Not every customer should be associated with a loan.
Customer
Loan
Borrows

18. Read this example
In DB for student registration we would like to keep data about students, courses, sections within courses and students' grades.
A course may include several sections.
Each one may be taught by different teacher.
A student may register in a course and be assigned to a particular section within this course

19. Model this problem using the ER approach
Data kept about each student include :name, address, telephone numbers, ID#, gender, and birth date.
For a course we would like to keep data concerning course-Id, course title, max. grade and number of lectures given/week.
Section's data include Section#, room number where lectures are given, teacher's name.
Section numbers are always 1, 2, ..etc. for each course.
The DB should keep the date when the student registered in the course, his final grade
Model this problem using the ER approach

20. Step 1:
The first step to extract the entities or tables form the last case as the following
Course
Student
Teacher
Section

21. Step 2
To extract the attributes inside in each Tables Note the types of attributes as discussed before (Simple, composite, Multivalve, derived,Key)

22. Step 3 Determine a primary key attribute for each table
The primary attribute is the one who are unique attribute and not allowed to be null value

23. Step 4
Read the case again and try to extract the relations between tables

24. Step 5
Take each relation in ERD and specify the type of participation for each table
If it is total participation or partial participation

25. Special cases

26. A Ternary Relationship
SID
JID
Quantity
supply
supplier
project
part
PID

27. Recursive Relationship type: Conversion (1:M)FN LN
1:1
EMPLOYEE
SUPERVISES
0:N SS

28. Binary versus non-binary relationship
customer
vehicle
owns
fixes
mechanic
parts

29. ER Advantages Exceptional conceptual simplicity Visual representation
Effective communication tool
Integrated with the relational data model

30. ER Disadvantages Limited constraint representation
Limited relationship representation
No data manipulation language
Loss of information content