Diagram visualization of the query
I understood the LEFT JOIN query, but I would like to make a comment:
I would only recommend, that instead of using the diagram to illustrate the results of the LEFT JOINT from the beginning, you would mention that the LEFT JOIN returns all results from the left table and matches with those of the right one, whether it exists on the right table or not. Then (and only then) by using the WHERE clause you can further refine your query in order to return only the results that belongs to the left table, which now is perfectly illustrate by the diagram used in this class.
It is just that we are led to think of the result of the LEFT JOIN (without the WHERE CLAUSE) as illustrate by the diagram, and that is not true.