How many instructors are there?
select count ( * ) from instructor
OK. Which instructor ids appear more than once in the advisor table
select t1.id from instructor as t1 join advisor as t2 on t1.id = t2.i_id group by t2.i_id having count ( * ) > 1
What are the instructors name for each of those ids the are advising 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