Entity in DBMS (Database Management System)

1. What is an Entity?

An entity in a Database Management System (DBMS) represents a real-world object that has a distinct identity. It can be anything that exists physically or logically and can be stored in a database.

Example:

• A Student in a university database.
• A Car in a vehicle registration system.
• A Book in a library management system.

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2. Types of Entities

1. Strong Entity

• An entity that can exist independently in the database.
• It has a primary key that uniquely identifies each record.

Example:

• Student (Student_ID, Name, Age) → Student_ID is the primary key.

Representation:

• Represented as a rectangle in the ER diagram.

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2. Weak Entity

• An entity that cannot exist independently and depends on a strong entity.
• It does not have a primary key, but it has a partial key.
• It is connected to a strong entity through a relationship (with total participation).

Example:

• Dependent (Dependent_Name, Age, Employee_ID) → Employee_ID is a foreign key.

Representation:

• Represented as a double rectangle in the ER diagram.

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3. Entity Set

• A collection of similar entities in the database.
• Example: All students in a university form a Student Entity Set.

Notation:

• Entity sets are represented by rectangles in an Entity-Relationship (ER) diagram.

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4. Attributes of an Entity

Each entity has attributes that define its characteristics.

Types of Attributes:

1. Simple (Atomic) Attribute: Cannot be divided further.
   • Example: Student_Name, Age.
2. Composite Attribute: Can be broken into smaller sub-attributes.
   • Example: Name (First Name, Last Name).
3. Derived Attribute: Derived from other attributes.
   • Example: Age (derived from Date of Birth).
4. Multivalued Attribute: Can have multiple values.
   • Example: Phone Numbers.
5. Key Attribute: Uniquely identifies an entity.
   • Example: Student_ID.

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5. Relationship Between Entities

Entities are connected through relationships in the database.

Types of Relationships:

1. One-to-One (1:1): Each entity in Set A is related to one entity in Set B.
   • Example: A Person has one Passport.
2. One-to-Many (1:M): One entity in Set A is related to many entities in Set B.
   • Example: A Teacher teaches many Students.
3. Many-to-Many (M:M): Many entities in Set A are related to many entities in Set B.
   • Example: Students enroll in multiple Courses.

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6. Entity vs. Table

Feature	Entity	Table
Definition	A real-world object with attributes.	A structured format to store entity data.
Representation	ER Diagram (Rectangle).	Rows & Columns in a database.
Example	Student entity with Student_ID, Name, Age.	Students table storing multiple records.

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7. Example ER Diagram

Scenario: University Database

• Entities:
  • Student (Student_ID, Name, Age)
  • Course (Course_ID, Course_Name)
• Relationship:
  • Student Enrolls in Course (Many-to-Many)

8. Conclusion

• An entity is a real-world object stored in a database.
• Strong entities exist independently, while weak entities depend on others.
• Entities have attributes, and they form entity sets.
• Relationships define how entities are connected.