Count the lessons grouped by staff first name.
select t2.first_name, count(*) from lessons as t1 join staff as t2 on t1.staff_id = t2.staff_id group by t2.first_name
How many lessons taught by staff whose first name has letter 'a' in it?
select count(*) from lessons as t1 join staff as t2 on t1.staff_id = t2.staff_id where t2.first_name like "%a%"