What are the first names of all professors teaching CIS-220?
select t1.emp_fname from employee as t1 join class as t2 on t1.emp_num = t2.prof_num where crs_code = 'CIS-220'
What are the first names of all professors teaching QM-261?
select t1.emp_fname from employee as t1 join class as t2 on t1.emp_num = t2.prof_num where crs_code = 'QM-261'
Who is teaching both?
select t1.emp_fname from employee as t1 join class as t2 on t1.emp_num = t2.prof_num where crs_code = 'CIS-220' intersect select t1.emp_fname from employee as t1 join class as t2 on t1.emp_num = t2.prof_num where crs_code = 'QM-261'