Answer
A primary key is a table constraint that uniquely identifies each row. • Its values must be unique and not null. • A table can have only one primary-key constraint, although it may contain several columns. • A primary key usually creates a unique index in PostgreSQL.
💡 SQL Example
CREATE TABLE users (user_id integer PRIMARY KEY, email text NOT NULL);
Result
CREATE TABLE
⚡ Quick Revision
A primary key uniquely and non-nullably identifies table rows.