Context: CREATE TABLE CLASS (prof_num VARCHAR); CREATE TABLE employee (emp_fname VARCHAR, emp_num VARCHAR)
Question: Find the first name of the professor who is teaching two courses with code CIS-220 and QM-261.
Answer: 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'