What are the ids of all the prerequisites?
select course_id from prereq
What is the id of the course for which Differential Geometry is a prerequisite?
select t1.course_id from prereq as t1 join course as t2 on t1.prereq_id = t2.course_id where t2.title = 'Differential Geometry'
What is its title?
select title from course where course_id in (select t1.course_id from prereq as t1 join course as t2 on t1.prereq_id = t2.course_id where t2.title = 'Differential Geometry')