How many pets are owned by students that have an age greater than 20?
SELECT count(*) FROM student AS T1 JOIN has_pet AS T2 ON T1.stuid  =  T2.stuid WHERE T1.age  >  20
SELECT count(*) FROM student AS T1 JOIN has_pet AS T2 ON T1.stuid  =  T2.stuid WHERE T1.age  >  20
[{'count(*)': 0}]