What are all the student GPAs?
select stu_gpa from student
Which of those are for students taking course ACCT-211?
select t2.stu_gpa from enroll as t1 join student as t2 on t1.stu_num = t2.stu_num join class as t3 on t1.class_code = t3.class_code where t3.crs_code = 'ACCT-211'
What is their average GPA?
select avg(t2.stu_gpa) from enroll as t1 join student as t2 on t1.stu_num = t2.stu_num join class as t3 on t1.class_code = t3.class_code where t3.crs_code = 'ACCT-211'