What is the minimum age?
select min(age) from person
Who has it?
select name from person where age = (select min(age) from person)
What is the average age?
select avg(age) from person
Who is younger than this?
select name from person where age < (select avg(age) from person)
Who calls these people friends?
select distinct t2.name from person as t1 join personfriend as t2 on t1.name = t2.friend where t1.age < (select avg(age) from person)