Primary Key

In relational databases like MySQL, every table should have a way to uniquely identify each row. The primary key provides this unique identifier.

A primary key is one or more columns in a table that uniquely identifies each row in that table.
No two rows can have the same primary key value.
A primary key cannot be NULL (it must always have a value).
Each table should have exactly one primary key.

Ensures uniqueness: No duplicate rows.
Provides a reliable way to reference rows (for example, when using foreign keys).
Improves data integrity.
Often used for indexing, which helps improve query performance.

CREATE TABLE students (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  age INT,
  email VARCHAR(150),
  enrollment_date DATE
);

In this example:

INSERT INTO students (id, name, age, email, enrollment_date)
VALUES (1, 'Alice Smith', 20, 'alice@example.com', '2025-06-23');

If you try to insert another row with id = 1, it will cause an error:

INSERT INTO students (id, name, age, email, enrollment_date)
VALUES (1, 'Bob Johnson', 22, 'bob@example.com', '2025-06-20');

Error: Duplicate entry ‘1’ for primary key.

A primary key can also consist of multiple columns — this is called a composite primary key.

Example:

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

In this example:

The combination of student_id and course_id uniquely identifies each row.
A student can enroll in multiple courses, but not in the same course more than once.

Often the primary key is a single column named id or tablename_id.
Many developers use auto-incrementing IDs so that the database automatically generates a new ID for each row (you will learn this in the next section).