What is a primary key?

A Primary Key is one or more fields (columns) in a database table that uniquely identify each record (row).

Main characteristics:

• Uniqueness: The value of the primary key must be unique for each row in the table.
• Not Null (NOT NULL): The primary key value cannot be NULL.
• Identification: Used to establish relationships between tables via foreign keys.
• Indexing: By default, the database automatically creates an index for the primary key to speed up search and join operations.

Example of creating a table with a primary key:

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,  -- The 'id' column is the primary key
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100)
);