And with no support of window functions, which also were not covered? Apart from looking to the solution right away, of course?
What’s the point of giving a practice exercise after the course if it by definition can’t be solved with the info from the course itself? 
Course: https://www.educative.io/collection/5352985413550080/6335716377231360
Lesson: https://www.educative.io/collection/page/5352985413550080/6335716377231360/5327494434521088
will someone from Educative clarify this please?
Hey, @Dmitry_Polovinkin!
I hope you’re doing well. Thank you for contacting us with your concerns. Our dedicated team is actively addressing the issue at hand. We have already initiated communication with the course author regarding the identified issues after the comprehensive review of the course.
We hope to see the fixes soon. We appreciate your patience and understanding as we work towards a solution. Please feel free to reach out if you have any further questions.