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.
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