Context: CREATE TABLE enroll (stu_num VARCHAR); CREATE TABLE student (stu_fname VARCHAR, stu_num VARCHAR)
Question: Find the first names of students who took exactly one class.
Answer: 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