CREATE TABLE Teachers (last_name VARCHAR)
CREATE TABLE Detention (teacher_id VARCHAR)

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
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"