What is primary key?

Answer 1

Maybe it helps you to read the section about primary keys in the course again.
Basic Concepts > Introduction to Databases, lesson 3.

Answer 2

Primary key is a unique identifier, such as a driver license number. Must cotains unique values, and cannot be null. Each table can have only one primary key, which may be using single or multiple fields

Answer 3

Primary key is a fuction that sets key to each table where it acts like the main character.

Answer 4

Primary key is basically the key used to uniquely identify the tuples in a database, it cannot contain any NULL values