1. Data Modelling Using Entity-Relationship (ER) Model
Chapter 3
Data Modelling Using Entity-Relationship (ER) Model

2. Phases of Database Design

3. “Company” Example Database Application
Description of the mini-world:
The company is organized into departments.
Each department has a unique name, a unique number and a particular employee who manages the department, including the date he/she began managing the department.
A department may have several locations.

4. Cont’d… A department controls a number of projects
Each project has a unique name, number and a single location

5. Cont’d…
We record each employee’s name, SSN, address, salary, sex and birth date. Also we record who is the supervisor
An employee works in one department but may work on several projects in different department
We track work hours of employees in projects

6. Cont’d
We record the dependants of each employee, including the first name, sex, birth date and relationship to the employee

7. ER Schema for “Company”

8. Entity & Attributes
Entity is the “thing” either physical existence (person, car, house) or conceptual (company, course, job).
Attributes are the properties of the entity
Each attributes has values.

9. Entity, attribute & value example

10. Composite vs. Simple Attributes
Composite attributes can be divided into smaller part.
For example, address consist of
Street Address
City
State
Postal Code
Simple (atomic) attributes are no longer divisble

11. Con’d

12. Single valued vs. Multi-valued
Singe valued:
Age
Sex
SSN
Multi valued:
Children

13. Other terms…
Stored vs. derived values. Derived values example may include GPA, sub-totals, etc.
Null values: “nothing”, not zero, not blank space!

14. Complex attributes
Composite and multi-valued attributes can be nested in an arbitrary way
() for nesting
{} for multi value
Example
{Address&Phone (
{Phone(AreaCode, PhoneNumber)},
Address(StreetAddress(Number, Street, ApartmentNumber),
City, State, Zip) ) }

15. Entity Type
It defines the set of possible entities with the same attributes
For example, ‘employee type’ is the set of employees in the company

16. Key Attributes of an Entity Type
What makes an entity unique?
An employee: SSN
A company: name
A project: number, name
A purchase slip may have 2 keys: date & time
Keys must be minimal

17. Value Sets Possible values of an attribute. Usually:
Numeric
Text
Boolean
Etc.
But can be more specific, for example:
Birth date must be > 1850 but before <(now)
Sex must be either female or male

18. Initial Entity Concept of Company Database

19. Relationship
Relationship type amongst entities defines a set of associations
For example between an employee and a department there is a relationship Works_For.
Represented as diamond shaped boxes

20. Degree of relationship type
Binary relationship
Ternary relationship

21. Binary rerlationship

22. Ternary Relationship

23. Recursive relationships

24. Constraints on relationships
Cardinality ratios
1:1, the relation Manages
M:N, the relation Works_On
N:1, the relation Works_FOR
Participation constraints
For example, an employee must be working for a department (null value is impossible)

25. M:N Relationship

26. 1:1 Relationship

27. Attribute of a relationship
Yes… a relationship may include additional attributes
For example
Start managing in the Manages relationship

28. Weak Entity Dependants do not have any key on its own
Why? Because there might be small chance that 2 employees have the same first name, birth date, sex for their son!
Personally, I do not like weak entities (hard to query).

29. The New ER Diagram

30. Summary of ER Diagram Notations

31. Cont’d…