How many followers does Mary have?
select followers from user_profiles where name = "Mary"
How about that of Susan?
select followers from user_profiles where name = "Susan"
Who is followed by either one of them?
select t3.name from user_profiles as t1 join follows as t2 on t1.uid = t2.f2 join user_profiles as t3 on t3.uid = t2.f1 where t1.name = "Mary" or t1.name = "Susan"
What are their ids?
select t2.f1 from user_profiles as t1 join follows as t2 on t1.uid = t2.f2 where t1.name = "Mary" or t1.name = "Susan"