Show me the last name of all teachers in file.
select last_name from teachers
How many teachers are involved in any detention?
select count(*) from teachers as t1 join detention as t2 on t1.teacher_id = t2.teacher_id
Give me the last name of teachers who are not one of those.
select last_name from teachers except select t1.last_name from teachers as t1 join detention as t2 on t1.teacher_id = t2.teacher_id