Find the first names of students who took exactly one class.
create table enroll (stu_num varchar, PRIMARY KEY (stu_num)); create table student (stu_fname varchar, stu_num varchar, PRIMARY KEY (stu_fname))
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