Show me last names of all the musicians who have been at the back position.
select t2.lastname from performance as t1 join band as t2 on t1.bandmate = t2.id where stageposition = "back"
Who has been at the back position the most?
select t2.lastname from performance as t1 join band as t2 on t1.bandmate = t2.id where stageposition = "back" group by lastname order by count(*) desc limit 1