A Primary Key is a fundamental concept in relational database management systems (RDBMS). It is used to uniquely identify each record (or row) in a database table.

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What is a Primary Key?

A Primary Key is a column or a set of columns in a table that uniquely identifies each row in that table.

Key Characteristics of a Primary Key:

1. Uniqueness: Each value in the primary key column(s) must be unique.
2. Not Null: A primary key column cannot contain NULL values.
3. Single Per Table: A table can have only one primary key.
4. Immutable: The value of a primary key should not change frequently.
5. Automatic Indexing: Most database systems automatically create an index on the primary key for faster access.

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📋 Example of Primary Key

Consider a table named Students:

student_id	name	age	class
101	Rahul Sharma	16	10-A
102	Neha Verma	15	9-B
103	Amit Singh	17	11-C

In this table:

• student_id is the Primary Key.
• It uniquely identifies each student in the table.
• No two students can have the same student_id.
• student_id cannot be NULL.

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SQL Syntax to Define a Primary Key

While creating the table:

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    name VARCHAR(100),
    age INT,
    class VARCHAR(10)
);

Or, define primary key separately:

CREATE TABLE Students (
    student_id INT,
    name VARCHAR(100),
    age INT,
    class VARCHAR(10),
    PRIMARY KEY (student_id)
);

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Composite Primary Key

A Composite Primary Key uses more than one column to uniquely identify a row.

Example:

CREATE TABLE CourseEnrollment (
    student_id INT,
    course_id INT,
    enrollment_date DATE,
    PRIMARY KEY (student_id, course_id)
);

In this case, neither student_id nor course_id alone is sufficient to identify a unique row. But together, they do.

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Violations of Primary Key Rules

student_id	name	age	class
101	Rahul Sharma	16	10-A
101	Neha Verma	15	9-B

• Duplicate student_id: Violates uniqueness.
• If student_id is NULL: Violates not-null property.

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Primary Key vs Unique Key

Feature	Primary Key	Unique Key
Uniqueness	Must be unique	Must be unique
NULL values	Not allowed	Allowed (one NULL only)
Number per table	Only one allowed	Multiple allowed
Purpose	Identifies records uniquely	Ensures uniqueness of column

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Summary

• A Primary Key ensures each record in a table is unique and not null.
• It can be a single column or a combination of columns.
• It is crucial for data integrity and relationships in relational databases.

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