Hello and welcome to the fourth part of our course. Today, we're going to introduce recursion and recursive CTEs, which is probably the most difficult thing we'll cover. Don't worry if you can't understand something at first – take your time and read the instructions carefully again and again.

Recursive CTEs make it possible to process 'hierarchical structures' – trees and graphs – using SQL. Such structures are used when talking about bosses and employees, railway connections, application menus, etc.

If you're ready to accept the challenge, let's move on.