Find the last names of teachers who are not involved in any detention.
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
last names of teachers not involved in detention refers to excluding teachers present in the Detention table from all teachers; EXCEPT is used to subtract one set of results from another; JOIN is used to combine rows from two or more tables based on a related column between them.