What are the id and first name of the student whose addresses have the highest average monthly rental?
SELECT T1.student_id ,  T2.first_name FROM Student_Addresses AS T1 JOIN Students AS T2 ON T1.student_id   =   T2.student_id GROUP BY T1.student_id ORDER BY AVG ( monthly_rental )  DESC LIMIT 1
highest average monthly rental refers to MAX(AVG(monthly_rental)); id and first name of the student refers to student_id and first_name; addresses' average monthly rental refers to monthly_rental in Student_Addresses; joining condition is T1.student_id = T2.student_id;