What is an auto-increment or serial column?

An auto-increment (MySQL) or SERIAL (PostgreSQL) column automatically generates a unique integer value for each new row, typically used for surrogate primary keys. The database maintains a sequence counter that increments by 1 with each insert. In MySQL: id INT AUTO_INCREMENT PRIMARY KEY. In PostgreSQL: id SERIAL PRIMARY KEY (shorthand for SEQUENCE + integer column + DEFAULT). In SQL Server: IDENTITY(1,1). The generated value is accessible after insert via LAST_INSERT_ID() (MySQL) or RETURNING id (PostgreSQL). Auto-increment values are never reused after deletion.