The Entity Relationship Data Model (ER) has existed for over 35 years.
The ER model is well-suited to data modelling for use with databases because it is fairly abstract and is easy to discuss and explain. ER models are readily translated to relations.
ER modelling is based on two concepts:
- Entities, that is, things. E.g. Prof. Ba, Course Database System.
- Relationships, that is, associations or interactions between entities.
E.g. Prof. Ba teaches course Database Systems ER models (or ER schemas) are represented by ER diagrams.
The example database
For the next section we will look at a sample database called the COMPANY database to illustrate the concepts of Entity Relationship Model.
This database contains the information about employees, departments and projects:
- There are several departments in the company. Each department has a unique identification, a name, location of the office and a particular employee who manages the department.
- A department controls a number of projects, each of which has unique name, a unique number and the budget.
- Each employee has name, an identification number, address, salary, birthdate. An employee is assigned to one department but can join in several projects.
- We need to record the start date of the employee in each project. We also need to know the direct supervisor of each employee. We want to keep track of the dependents of the employees. Each dependent has name, birthdate and relationship with the employee.