Which courses have some prerequisite?
select * from course as t1 join prereq as t2 on t1.course_id = t2.course_id
Which ones have two prerequisites among them?
select * from course as t1 join prereq as t2 on t1.course_id = t2.course_id group by t2.course_id having count(*) = 2
What are their titles?
select t1.title from course as t1 join prereq as t2 on t1.course_id = t2.course_id group by t2.course_id having count(*) = 2