Introduction of ER Model

An ER Model is an Entity Relationship Model that visualizes data relationships in a Database. It is a part of the DATA Model. It is a structural format of the database used to model real-world objects for example a student, a teacher, or a school, and the relation between these real-world objects.
An ER diagram is a visual representation of the relationships between entities in a database. It is used in database design to show the structure of the database.
ER Diagram was developed by Peter Chen in 1976. The ER model was created as a simple-to-understand model to represent the structure and logic of the database.
The entity relational model is a model for identifying the entities represented in a database and representing how those entities are related. The ER data model specifies the enterprise schema that graphically represents the overall logical structure of the database.

ER Model Symbols

ER Model symbol is used to represent a relationship between two objects. This is a logical view of a database object model.

Main Components of ER Diagram

1. Entity                  2. Attributes            3. Relationship

1. Entity

Entities represent objects or concepts relevant to the database design. An entity can be a physical object for example a person, student, car, house, or employee, or an ideological object for example a company, or a job.

Nature of Entity-

1. Tangible Entity –One can physically touch or see it. For example, a product, item, car, or bike, often have straightforward attributes.
2. Intangible Entity – It is something that can not be physically touched or seen. It represents abstract concepts or ideas for example courses etc. it represents a transaction or process

Types of Entity

1. Strong entity:

You can uniquely identify a strong entity by its attributes. It does not rely on any other entity for its identification. We also call it an independent entity.
For example, the Student entity has attributes like StudentID, Name, and DOB. Here, the StudentID uniquely identifies each student, making the entity distinct based on the StudentID.
a rectangle represents it.

2. Weak Entity:

A weak entity cannot uniquely identify itself using its attributes alone. It relies on a strong entity to provide part of its identifying information. this have a partial key and their existence depends on the strong entity.
For Example, The company can collect information about employees’ family members children, wife. But without the employee dependents can’t exist. So there will be a dependent is a weak entity, and employees will be a strong entity.
it is represented by a double rectangle.

3. Derived Entity:

This entity derives its data from other entities. It does not store data directly but instead obtains it from the values of other entities.
For example Age entity which derived from DateofBirth and CurrentDate.

4. Associative Entity

This represents a relationship between two or more entities, especially when the relationship itself has attributes. Most often used in many-to-many relationships. it is represented by Double diamond.
Example: Entity ‘CourseEnrollment‘
Attributes: EnrollmentDate and  Grade
Represents the relationship between Student and Course

5. Composite Entity

To model many-to-many relationships between entities, we use composite entities by decomposing them into a combination of entity types and their attributes. For Example,
Entity: OrderDetail
Attributes: OrderID, ProductID, Quantity, Price
Represents the many-to-many relationship between Order and Product with additional details like Quantity and Price.

6. Subtypes and Supertypes

Model hierarchical relationships by using supertypes as general entities and subtypes as specific entities that utilize supertypes.
Supertype: Person
Attributes: PersonID, Name, DateOfBirth
Subtypes: Student, Faculty
Student and Faculty inherit common attributes from Person but have additional specific attributes.

7. Multi-Valued Entity

A multi-valued entity has attributes that can have multiple values ​​for a single entity instance. These attributes are not limited to a single value.
For Example, An employee Entity has a skill attribute that can be more than one skill in an employee.

2.Attributes

Attributes are the properties or characteristics of an entity or relationship that define the entity type. Attributes describe specific details about entities or relationships and define the data that a database will store.
For example, Student Roll_No, Name, DOB, Age, and Class are the attributes that define entity type Student
An oval represents the attribute.

Types of Attributes

1. Key Attribute:-  that attribute which uniquely identifies an entity in an entity set that is called a key attribute.
For example, employee table empid is a key attribute.
The key attribute is represented by an oval with underlying lines.

2. Composite Attribute

This attribute is composed of many other smaller attributes, each of which represents more specific details.
For Example, Address is composed of Street, City, State, and ZipCode. all represent by oval shape only.

3. Multivalue Attributes

An attribute is composed of more than one value for a given entity.
For example, Phoneno. a person may have more than one phone no.

It is represented by a double oval.

4. Derived Attribute

A derived attribute is an attribute that we can derive from other attributes of the entity type.
For Example, DOB and Current date resulted in Age.
A dashed oval represents a derived attribute.

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