What is the first names of the professors from the history department who do not teach a class.
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
first names = emp_fname; professors from the history department = JOIN operation between employee, professor, and department tables with condition dept_name = 'History'; do not teach a class = usage of EXCEPT to exclude professors who are listed in the CLASS table.