What is a primary key in SQL?

Every table typically has one primary key, which the database uses to enforce that no two rows are identical and to quickly locate specific rows.

Key properties

• Uniqueness: no two rows can share the same primary key value
• Not null: a primary key column can never be empty
• Indexed automatically: most databases build an index on the primary key, making lookups by that key fast

Primary key vs. foreign key

A primary key uniquely identifies rows within its own table. A foreign key is a column in one table that references a primary key in another table, creating a relationship between the two — for example, an "orders" table might have a customer_id foreign key referencing the primary key of a "customers" table.