mysql group every n row

by the way, you can calculate the data like for every  5 minutes ( in database each row  for 20 seconds interval ).   example below : grouped  every  5 row in series as group then you can make function like math avg , count etc.

set @i :=0;
SELECT *, @i:=@i+1 as rownum, FLOOR(@i/5) AS `datagrp` FROM table_name_muslum21