What is a primary key?

A primary key is a column or a set of columns in a database table that uniquely identifies each row in that table.

Main characteristics:

• Uniqueness: The primary key values must be unique for each row.
• Not null (NOT NULL): The primary key value cannot be NULL.
• One per table: Each table can have only one primary key.
• Indexing: When creating a primary key, the database automatically creates an index for it to improve search and join performance.

Example:

In the users table, the primary key can be the id column:

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY, // Defining the primary key
    username VARCHAR(255) NOT NULL UNIQUE,
    email VARCHAR(255) NOT NULL UNIQUE
);

Uses of primary key:

• Record identification
• Linking tables through foreign keys
• Ensuring data integrity