How to use order by with modified column - sql

Question

There is a Table with a age-column. The Value of this column is a varchar because the age of persons under the age of 1 is saved in months with an additional 'm' (9 Month old -> '9m')
I know that this is generally a bad idea and one should rather persist the date of birth, but in this case the age refers to the age on a specific day in history - and additionally this is part of a lesson and the whole point is learning how to treat "weird" data.
My first idea was to put a leading zero on all ages which are not purely numeric:
SELECT *
FROM db
ORDER BY REPLACE(age, (IF ISNUMERIC(age) age ELSE CONCAT('0', age))) DESC;
However this is not a valid SQL-statement and neither are my other attempts.
The question is: How can I adjust the value used for ORDER BY without altering the db?
Another approach would be to select only the rows with a purely numeric age value and a separate select for the remaining rows order both of them separately and combine them afterwards.
My take on this was the following:
(SELECT name, age
FROM titanic
WHERE ISNUMERIC(age)
ORDER BY age DESC)
UNION
(SELECT name, age
FROM titanic
WHERE NOT ISNUMERIC(age)
ORDER BY age);
This is in fact valid or at least it gives me a result. But in the result I can't really see what happened to the order, it looks like the UNION undos everything.
Thanks in advance, will take any tip or even just the name of the function/method I should look into!

Answer 1

Would this work?
SELECT name, age
FROM titanic
ORDER BY isnumeric(age), age

Answer 2

I would use "Case When" structure with some transformations in the "Order by" to get the total number of months for both types of ages.
Select name, age
From tbl
Where age SIMILAR TO '[1-9][0-9]*' Or
age SIMILAR TO '[1-9][0-2]?m'
Order by Case When age SIMILAR TO '[1-9][0-2]?m' Then Substring(age,1,CHAR_LENGTH(age)-1)::int
When age SIMILAR TO '[1-9][0-9]*' Then age::int * 12 End

Answer 3

First off storing age is a poor idea, you must update every row regularly, in this case at least monthly. Storing it as string turns a poor idea into a terrible idea. Not only do you have the maintenance, it is not straight forward nor is it straight forward to insert it. Instead store date-of-birth as a timestamp or date. You can then quickly get age (via the age) you can then use the extract function on the resulting interval at whatever level is desired. If you absolutely must present age with text indicator for either years/months (or even days) you create a view that derives the appropriate value. (see demo)
create or replace view titanic as
select name "Name"
, case when extract( year from age(dob))::integer > 0 then to_char (extract( year from age(dob)), '999') || ' years'
when extract( month from age(dob))::integer> 0 then to_char (extract( month from age(dob)), '99') || ' months'
else to_char (extract( day from age(dob)),'99') || ' days'
end "Age"
from titanic_tbl;