What are the first names of all professors who teach more than one class?
SELECT T2.emp_fname FROM CLASS AS T1 JOIN employee AS T2 ON T1.prof_num  =  T2.emp_num GROUP BY T1.prof_num HAVING count(*)  >  1
first names of all professors = emp_fname; professors who teach more than one class = HAVING COUNT(*) > 1; professors = employee; classes taught by professor = JOIN on professor identifier;