A primary key (PK) is a unique identifier for each record in a database table.
- Uniqueness: No two records can have the same primary key value.
- Non-null: A primary key cannot contain null values; every record must have a valid primary key.
- Immutability: Ideally, the primary key should not change over time, as it serves as a stable reference for the record
For example, an ISBN serves as a primary key for books, uniquely identifying each book in the database