What is the average age of the students living in each dorm?
SELECT avg ( T1.age )  ,  T3.dorm_name FROM student AS T1 JOIN lives_in AS T2 ON T1.stuid   =   T2.stuid JOIN dorm AS T3 ON T3.dormid   =   T2.dormid GROUP BY T3.dorm_name
average age of students = AVG(age); each dorm refers to GROUP BY dorm_name; students living in a dorm involves JOIN between student and dorm tables through a relationship table (lives_in).