codice:
set @attr1 = -1,@num = 1;
select attr1,avg(attr2) as media from (
	select *,@num := if(@attr1 = attr1, @num + 1, 1) as riga,
	@attr1 := attr1 as i from (
		select id,attr1,attr2 from tabella order by attr1,id
		) as t 
	) as t2
group by attr1,ceil(riga/5)
order by attr1,id