Question: What are the first names of all history professors who do not teach?
SQL: SELECT T1.emp_fname FROM employee AS T1 JOIN professor AS T2 ON T1.emp_num  =  T2.emp_num JOIN department AS T3 ON T2.dept_code  =  T3.dept_code WHERE T3.dept_name  =  'History' EXCEPT SELECT T4.emp_fname FROM employee AS T4 JOIN CLASS AS T5 ON T4.emp_num  =  T5.prof_num