Gruppér rækker efter årbåndsinterval

Beregn først tiåret for hver række

select floor(year(`year`) / 10) * 10 as decade
from tbl_people

og brug derefter dette mellemresultat til at tælle SQL Fiddle

select count(*), decade, decade + 9
from (select floor(year(`year`) / 10) * 10 as decade
      from tbl_people) t
group by decade

eller denne SQL violin , hvis du vil have årtiet i én kolonne

select count(*) as count, concat(decade, '-', decade + 9) as year
from (select floor(year(`year`) / 10) * 10 as decade
      from tbl_people) t
group by decade