Question: What is the difference in the average GPA of students who took the hardest and easiest courses?
SQL: SELECT AVG(T1.gpa) FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T3.diff IN (2, 1) GROUP BY T3.diff