Introduction

The ER model models a database in the form of its entities and the relationships between those entities.

An entity is an object that exists and is distinguishable from other objects.

Entities have attributes. The domain of the attribute specifies the set of values that the attribute can hold. Attributes can be:

• simple or composite

• single-valued or multivalued attributes

• derived (can be computed from other attributes)

An entity set is a set of entities of the same type that share the same properties.

A relationship is an association among several entities.

A relationship set is a mathematical relation among n ≥ 2 entities, each taken from entity sets.

Relationships can be binary (between 2 entities) or ternary (between 3 entities).