Question: Calculate the difference between the average satisfaction of the students with high salaries and no salary.
SQL: SELECT AVG(T2.sat) - ( SELECT AVG(T2.sat) FROM RA AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id WHERE T1.salary = 'free' ) AS diff FROM RA AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id WHERE T1.salary = 'high'