MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table test (`value` decimal(10,1));
insert into test (`value`) values
(1),
(5),
(10.5),
(12),
(36);
SELECT Ranges.`range`, COALESCE(AVG(Q.`value`), 0) as `avg`
FROM
(
  SELECT 0 as `class`, ' 0-21' as `range`
  UNION ALL SELECT 21, '21-34'
  UNION ALL SELECT 34, '34-64'
  UNION ALL SELECT 64, '64+'
) Ranges
LEFT JOIN
(
  SELECT 
   `value`,
   case
   when `value` between  0 and 21 then 0
   when `value` between 21 and 34 then 21
   when `value` between 34 and 64 then 34
   when `value` > 64 then 64
   end as rangeclass
  FROM test
) Q ON Q.rangeclass = Ranges.`class`
GROUP BY Ranges.`range`
ORDER BY Ranges.`range`


range
avg


 0-21
7.12500

21-34
0.00000

34-64
36.00000

64+
0.00000