MySQL 8.0

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select version();

version()

8.0.36

create table cars (id serial, content json);
insert into cars (content) values ('{"a":1,"b":2}'),('{"b":3,"c":4}');

Records: 2  Duplicates: 0  Warnings: 0

select distinct `key`
from cars
cross join json_table(
  json_keys(cars.content),
  '$[*]' columns(`key` text path '$')
) `keys`

key

a

b

c