What are the names of all instructors who advise students?
select t1.name from instructor as t1 join advisor as t2 on t1.id = t2.i_id
Of those, which advise more than one student?
select t1.name from instructor as t1 join advisor as t2 on t1.id = t2.i_id group by t2.i_id having count(*) > 1