List all volumes.
select * from volume
Only show information for volumes associated with artists aged 25 or younger.
select * from artist as t1 join volume as t2 on t1.artist_id = t2.artist_id where t1.age <= 25
What is the average weeks on top for these volumes?
select avg(t2.weeks_on_top) from artist as t1 join volume as t2 on t1.artist_id = t2.artist_id where t1.age <= 25