Hello and welcome to the fourth part of our course. Today, we're going to introduce the concepts of recursion and recursive CTEs; this is probably the most difficult part of our entire course. 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 T-SQL. For instance, such structures are used when talking about superior and inferior employees, railway connections, application menus, etc.

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