Database Management Systems (dbms)

should have employee (Total)

M:1
Employee
Department
Assigned to
M
1
Works on
Project
N
M

Step Two - Identify relationship types..

• “An employee may have dependents.”

Employee
Dependent
Has dependant
1
M

Step Two - Identify relationship types..

• “An employee may or may not supervise another employee.”
• Assumptions:
• Supervision relationship is known as recursive relationship. Employee play two roles: Supervisor , Supervisee

Employee
1
M
supervision
supervisor
supervisee

E-R Model (schema) Diagram for the company database

EMPLOYEE
Assigned to
DEPARTMENT
Supervision
Manages
Works on
Project
Controls
DEPARTMENT
Has dependant
N
N
N
N
N
1
1
1
1
1
1
M

THE RELATIONAL DATA MODEL

Introduction

• This model was introduced by Ted Codd of IBM in the early 1970’s.
• Provide concepts which can be used to build a logical design.
• It is based on the concept of mathematical relations.
• DBMS which supports this model is said to be a Relational Database Management System (RDBMS)
• The RDB is a collection of relations.

Terminology

• Relation
• A relation is a table with columns and rows.
• Attribute
• An attribute is a named column of a relation.
• Tuple
• A tuple is a row of a relation.