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Entity-Relationship Model


To facilitate Database Design by allowing specification of an Enterprise Schema that represents the overall logical structure of a database is called as Entity-Relationship Model


Entity Sets :


An ‘Entity’ is a ‘thing’ or ‘object’ in the real world that is different from all other objects.

For example, each employee in a company is an entity.


An entity has a set of properties, and the values for some properties in a set may be uniquely identified as an entity.

For example, The property “student-id” uniquely identifies a person.


An ‘Entity set’ is a set of entities of the same type that share the same properties or attributes.

The actual collection of the entities belonging to the entity set is called ‘Extension’.


Attributes : A set of attributes represents an entity. Each of an entity’s attributes has a value.

Domain : The set of all values of an attribute is called its domain.


Relationship Sets:

A relationship is an association between two or more entities.

Relationship set: A set of similar types of relationships. Here, the entities are said to be a part of the relationship.

A relationship instance in the E-R schema represents an association between the named entities.

If an entity has an association with the same entity, then it is called ‘Recursive Relationship’.

A relationship may have additional attributes, known as ‘descriptive attributes’.

Ternary Relationship: A relationship with three entities is called a ‘ternary relationship’ 




For every attribute, there is a permitted set of values, known as ‘Domain’ or ‘Value Set’.


Simple Attribute: It is said to be a simple attribute when the value cannot be divided. 


Composite Attribute: It is said to be a composite attribute when the value can be divided. 


Single-valued Attributes: The attribute has only one value.


Multi-Valued Attributes: The attribute has several values.


Derived Attribute: The value of this attribute can be derived from the value of another attribute 


Entity relationship model of database.