How many courses is each student enrolled in?
select count(*), t1.stu_num from student as t1 join enroll as t2 on t1.stu_num = t2.stu_num group by t1.stu_num
Which students are enrolled in only one?
select * from student as t1 join enroll as t2 on t1.stu_num = t2.stu_num group by t2.stu_num having count(*) = 1
What are their first names?
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