Find the first names of students who took exactly one class.
SELECT T1.stu_fname FROM student AS T1 JOIN enroll AS T2 ON T1.stu_num  =  T2.stu_num GROUP BY T2.stu_num HAVING count(*)  =  1
first names of students = stu_fname; exactly one class refers to HAVING COUNT(*) = 1; students who took class refers to JOIN between student and enroll tables on student number (stu_num);