CREATE TABLE CLASS (prof_num VARCHAR)
CREATE TABLE employee (emp_fname VARCHAR, emp_num VARCHAR)

Find the first names of professors who are teaching 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
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