Group by age range mysql

I'm trying to count the number of people by age ranges, and I can almost do it with 2 problems:

1. If there are no people in a given age range (NULL), then that age range does not appear in the results. For example, in my data there's no entries for "Over 80" so that date range does not appear. Basically, it looks like a mistake in the programming when there are missing date ranges.

2. I'd like to order the results in a specific way. In the query below, because the ORDER BY is by age_range, the results for '20 - 29' come before the results for 'Under 20'.

Here's a sample of the db table "inquiries":

inquiry_id  birth_date
1           1960-02-01
2           1962-03-04
3           1970-03-08
4           1980-03-02
5           1990-02-08

Here's the query:

SELECT
    CASE
        WHEN age < 20 THEN 'Under 20'
        WHEN age BETWEEN 20 and 29 THEN '20 - 29'
        WHEN age BETWEEN 30 and 39 THEN '30 - 39'
        WHEN age BETWEEN 40 and 49 THEN '40 - 49'
        WHEN age BETWEEN 50 and 59 THEN '50 - 59'
        WHEN age BETWEEN 60 and 69 THEN '60 - 69'
        WHEN age BETWEEN 70 and 79 THEN '70 - 79'
        WHEN age >= 80 THEN 'Over 80'
        WHEN age IS NULL THEN 'Not Filled In (NULL)'
    END as age_range,
    COUNT(*) AS count

    FROM (SELECT TIMESTAMPDIFF(YEAR, birth_date, CURDATE()) AS age FROM inquiries) as derived

    GROUP BY age_range

    ORDER BY age_range

Here's a simple solution based on the suggestion by Wrikken:

SELECT
    SUM(IF(age < 20,1,0)) as 'Under 20',
    SUM(IF(age BETWEEN 20 and 29,1,0)) as '20 - 29',
    SUM(IF(age BETWEEN 30 and 39,1,0)) as '30 - 39',
    SUM(IF(age BETWEEN 40 and 49,1,0)) as '40 - 49',
    SUM(IF(age BETWEEN 50 and 59,1,0)) as '50 - 59',
    SUM(IF(age BETWEEN 60 and 69,1,0)) as '60 - 69',
    SUM(IF(age BETWEEN 70 and 79,1,0)) as '70 - 79',
    SUM(IF(age >=80, 1, 0)) as 'Over 80',
    SUM(IF(age IS NULL, 1, 0)) as 'Not Filled In (NULL)'

FROM (SELECT TIMESTAMPDIFF(YEAR, birth_date, CURDATE()) AS age FROM inquiries) as derived

Dear friend, I help about this command below

SELECT CASE WHEN `p_age` < 18 THEN 'under 18' WHEN `p_age` 
BETWEEN 18 and 29 THEN '18-29' WHEN `p_age` 
BETWEEN 30 and 39 THEN '30-39' WHEN `p_age` 
BETWEEN 40 and 49 THEN '40-49' WHEN `p_age` 
BETWEEN 50 and 59 THEN '50-59' WHEN `p_age` 
BETWEEN 60 and 69 THEN '60-69' WHEN `p_age` 
BETWEEN 70 and 79 THEN '70-79' WHEN `p_age` > 80 
THEN 'Over 80' END as RANGE_AGE, count(`id`) as COUNT FROM `tb_people_quarantine` 
GROUP BY RANGE_AGE ORDER BY RANGE_AGE";

and the result is below

The result I want it will show 'OVER 80'      0

the same as other range age, if they are no information should show 0

Two options.

a) Use sub query

select 
  concat(10*floor(age/10), '-', 10*floor(age/10) + 10) as `range`, 
  gender, 
  count(*) as count 
from (
  select 
    *, 
    TIMESTAMPDIFF(YEAR,birth_year,CURDATE()) AS age
  from 
    transaction left join patient on patient_id = patient.id
) as t group by `range`, gender;

Gives

+-------+--------+-------+
| range | gender | count |
+-------+--------+-------+
| 0-10  | m      |     2 |
| 10-20 | m      |     1 |
| 20-30 | m      |     3 |
| 30-40 | f      |     2 |
| 30-40 | m      |     2 |
| 50-60 | f      |     1 |
| 50-60 | m      |     1 |
| 60-70 | f      |     5 |
| 60-70 | m      |     3 |
| 70-80 | f      |     1 |
| 70-80 | m      |     6 |
+-------+--------+-------+

b) Group by age first and then group in range in application.

When using ORMs it's not easy to do sub queries, so this might be a good idea.

Performance wise: Considering that average humans live for 100 years, there can be at max 200 records (100 male and 100 female) which can be easily looped in an application.

select 
  gender, 
  TIMESTAMPDIFF(YEAR,birth_year,CURDATE()) AS age,
  count(*)
from 
  transaction left join patient on patient_id = patient.id
group by age, gender

Gives

+--------+------+----------+
| gender | age  | count(*) |
+--------+------+----------+
| m      |    4 |        2 |
| m      |   14 |        1 |
| m      |   24 |        3 |
| f      |   34 |        2 |
| m      |   34 |        2 |
| f      |   54 |        1 |
| m      |   54 |        1 |
| f      |   64 |        5 |
| m      |   64 |        3 |
| f      |   74 |        1 |
| m      |   74 |        6 |
+--------+------+----------+