Question: Find the number of students who is older than 20 in each dorm.
SQL: SELECT count(*) ,  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 WHERE T1.age  >  20 GROUP BY T3.dorm_name