A primary key in a relational database is a unique identifier for each record in a table. It ensures that each row has a distinct and non-null value, allowing for efficient data retrieval and maintaining data integrity. The primary key can be a single column or a combination of columns, known as a composite key.
Every record in a table is uniquely identified by the PRIMARY KEY constraint. Primary keys cannot have NULL values in them; instead, they must have UNIQUE values. There can only be ONE primary key per table, and it can include one or more columns (fields) in the key.