CREATE TABLE employee (emp_fname VARCHAR, emp_jobcode VARCHAR)
CREATE TABLE CLASS (prof_num VARCHAR)

What are the first names of the professors who do not teach a class.
SELECT emp_fname FROM employee WHERE emp_jobcode = 'PROF' EXCEPT SELECT T1.emp_fname FROM employee AS T1 JOIN CLASS AS T2 ON T1.emp_num = T2.prof_num
SELECT "emp_fname" FROM "employee" WHERE "emp_jobcode" = 'PROF' EXCEPT SELECT "T1"."emp_fname" FROM "employee" AS "T1" JOIN "CLASS" AS "T2" ON "T1"."emp_num" = "T2"."prof_num"