What is a primary key?

Answer 1

a primary key is a unique identifier, for each data set in your table. I can't be null and it's much easier to work with your data if you can identify each single row.

Answer 2

A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields.