What are all the ids of the prerequisite classes?
select prereq_id from prereq
Of these, which is the prerequisite for Mobile Computing?
select t1.prereq_id from prereq as t1 join course as t2 on t1.course_id = t2.course_id where t2.title = 'Mobile Computing'
What is its title?
select title from course where course_id in (select t1.prereq_id from prereq as t1 join course as t2 on t1.course_id = t2.course_id where t2.title = 'Mobile Computing')