tried this on postgresql

--Syntax
SELECT 
LOWER(REGEXP_REPLACE(<CamelCaseExample>, '([a-z0-9])([A-Z])', '\1_\2', 'g')) AS snake_case;

--Example
SELECT 
LOWER(REGEXP_REPLACE('CamelCaseExample', '([a-z0-9])([A-Z])', '\1_\2', 'g')) AS snake_case;

--On all records in any table
with my_table (camelcase) as (
  values ('StackOverflow'), 
         ('Stackoverflow'), 
         ('stackOverflow'), 
         ('StackOverFlowCom'), 
         ('stackOverFlow')
)
select camelcase, trim(both '_' from lower(regexp_replace(camelcase, '([a-z0-9])([A-Z])', '\1_\2', 'g'))) as snake_case
from my_table;

Just FYI, a) REGEXP_REPLACE finds places where a lowercase letter or digit is followed by an uppercase letter. b) It inserts an underscore (_) between them. c) LOWER() converts the entire result to lowercase.

output