Primary Key

A primary key uniquely identifies each row in a table and is defined by a primary key constraint. It must be unique and cannot contain NULL values. Every table should have one primary key to ensure each record is identifiable.

For example, a user table can have a unique id that serves as the primary key:

CREATE TABLE users (
  id   INT PRIMARY KEY,
  name VARCHAR(100)
);
-- 'id' must be unique and non-null for every row.