CREATE TABLE default.json_test (tags JSON) ENGINE = Memory;

INSERT INTO default.json_test (tags) VALUES 
  ('{"a":"b"}'),
  ('{"c":{"d":"e"},"f":{"g":"h"}}')
;

SELECT
    property_name,
    JSONExtractString(toString(tags), 'String') AS value
FROM default.json_test
ARRAY JOIN JSONAllPaths(tags) AS property_name

-- Empty values
   ┌─property_name─┬─value─┐
1. │ a             │       │
2. │ c.d           │       │
3. │ f.g           │       │
   └───────────────┴───────┘

SELECT
    a.1 AS path,
    a.2 AS value
FROM default.json_test
ARRAY JOIN JSONExtractKeysAndValues(toString(tags), 'String') AS a

-- No "nesting"
   ┌─path─┬─value─────┐
1. │ a    │ b         │
2. │ c    │ {"d":"e"} │
3. │ f    │ {"g":"h"} │
   └──────┴───────────┘

SELECT
    path,
    getSubcolumn(tags, path) AS value
FROM default.json_test
ARRAY JOIN JSONAllPaths(tags) AS path

Received exception from server (version 25.4.7):
Code: 44. DB::Exception: Received from localhost:9000. DB::Exception: The second argument of function getSubcolumn should be a constant string with the name of a subcolumn: In scope SELECT path, getSubcolumn(tags, path) AS value FROM default.json_test ARRAY JOIN JSONAllPaths(tags) AS path. (ILLEGAL_COLUMN)