Components Of E-R Model:

1. Entity: 

An Entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in E-R diagrams by a Rectangle and Named using singular Nouns.

Two Types of Entity:

• Weak Entity: An Entity type that has no key attribute is called Weak Entity Type. It is also called Child or Subordinate Entity Type represented by .
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• Strong Entity: An Entity Type that have key attribute is called Strong Entity Type. e.g. in below example Employee is Strong Entity Type.
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Some Entity Types have more than one key attribute. They are called Composite Key Attributes.

2. Attribute: 

Entities are represented by means of their properties, called Attributes. All Attributes have values. For Example, a ‘Student’ Entity may have name, class, and age as Attributes.

There exists a domain or range of values that can be assigned to Attributes. For Example, a ‘student’s name’ cannot be a numeric value. It has to be alphabetic. A ‘Student’s age’ cannot be Negative, etc.

Types of Attributes:

• Key Attribute: The Attribute which uniquely identifies each Entity in the Entity Set is called Key Attribute. For Example, ‘Roll No.’ will be unique for each Student. In E-R Diagram, Key Attribute is represented by an Oval with Underlying Lines. 

• Composite Attribute: Composite Attributes are made of more than one Simple Attributes. For Example, a student’s complete name may have ‘First_Name’ and ‘Last_Name’.
• Multivalued Attribute: It is defined as an Attribute that holds Multiple Values for a Single Entity. For Example, for a ‘Student’ Entity, if one of the Attributes is ‘Hobby’ then ‘Hobby’ can have multiple values like Reading, Dancing, Singing, and so on.
• Derived Attribute: Derived Attributes are the Attributes that do not exist in the Physical Database, but their values are Derived from other Attributes present in the Database. For Example, ‘average_salary’ in a department should not be saved directly in the Database, instead it can be Derived. For another Example, age can be Derived from Date of Birth.

3. Relationship: 

A Relationship is association among several Entities. For Example, there is a relationship between Students and Teachers. The Relationship represents the fact that a Teacher teaches several Students and a Student is taught by Several Teachers. The Relationship can be named as Teaching.

Types of Relationships:

• One-to-One (1:1): In One-to-One Relationship, If an Entity in Entity Set X is associated with at most one Entity in Set Y, and an Entity in Y is assosiated with at most one Entity in X then then the Relationship is known as One-To-One Relationship.

For example, in a university, if we consider two entity sets as ‘Teachers’ and ‘Departments,’ and they are related by the ‘Head of the Department’ relationship, then there is a one-to-one mapping. This is because one department can be headed by only one teacher, and conversely, one teacher can be the head of only one department.

• Many-to-One (n:1):  If an entity in the set X is associated with at most one entity in set Y and an entity in set Y is however associated with any number of entities in set X then such a relationship is known as many to one Relationship.

In this example, Diagram below, two entity sets teacher and department associated by a relation member is in a many to one relationship. As there can be many teachers belonging to one Department.

But one teacher cannot be associated with more than one department. Thus, it is a many-to-one Relationship.

• One-to-Many (1:n):  If an entity set X is associated with any number of entities in set Y and an entity in set Y is however associated with at most one entity in set X. Then we say that there is one-to-many Relationship between X and Y. It shows the logic of one-to-many Relationship.

The E-R diagram represents a one-to-many relationship, where there are two entity sets: one for teachers and the other for subjects. In this scenario, a teacher can teach many subjects, but it is assumed that each subject will be taught by only one teacher, establishing a one-to-many relationship.

• Many-to-Many (n:n):  If an entity in set X associated with any number of entities in set ‘Y and an entity in set Y is associated with any number of entities set X then it makes a many to many relationships.

In a college, there are multiple teachers, each capable of teaching multiple subjects, and conversely, one subject can be taught by multiple teachers. This establishes a many-to-many relationship between the two entity sets: teachers and subjects.