Kelompokkan baris berdasarkan Interval Band Tahun

Hitung dulu dekade untuk setiap baris

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

dan kemudian gunakan hasil antara ini untuk menghitung SQL Fiddle

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

atau SQL Fiddle , jika Anda ingin dekade dalam satu kolom

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