What is a primary key?

A primary key (PK) is a column (or combination of columns) that uniquely identifies each row in a table. Primary key constraints: uniqueness (no two rows can have the same PK value) and NOT NULL (PK columns cannot be null). A table can have only one primary key. Types: natural key — a real-world attribute (e.g., email), and surrogate key — an artificial identifier (e.g., auto-increment integer id or UUID). Surrogate keys are preferred in practice because natural keys can change and are often longer, making joins less efficient.