Entity Relationship Essays Example

Other Details

Entity Relationship
Introduction
Entity relationship or ER is the relation between entities within a software system. An entity may be described as a group of related data. In the present case we have an employee table which has a relation with itself. A manager – who is also an employee – manages a group of employees. Such a relationship is called a recursive or unary relationship. We have been tasked with depicting and explaining such a relationship (Chen, 1976).

Model showing the following EMPLOLYEE relationships:

An employee must have a manager
N

Manages

1

A manager may have at least 1 employee

N

Manages

1

A manager must have at least 1 employee

1
Manages
1

A manager may have no employees

Manages
1

Model showing the following PROJECT – EMPLOYEE relationships

One employee can participate in many projects
1 N

One project can have many employees

N 1

Questions

Which type of entity will help you model this many-to-many relationship?
M N

Is it possible for a project to have no employees assigned, and how would you model that possibility?

It would not be possible for a project to have no employees in a real life situation. This situation could be modelled in a database by showing that a project must have at least 1 employee. This would be modelled as under:
N 1

Is it possible for an employee to be unassigned to any project, and how would you model that possibility?

It is not possible for an employee to be unassigned to any project since a project must have at least 1 employee. This eventuality would be prevented by the PROJECT-ID FK in the employee table. The relationship would be modelled as under:
N 1

References

Chen, Peter (March 1976) "The Entity-Relationship Model - Toward a Unified View of Data" ACM Transactions on Database Systems 1 (1): 9–36 doi:10.1145/320434.320440