How to extract keys from a JSON (JavaScript Object Notation) object using the JSON_KEYS() function?

JSON_KEYS(json, path) is a MySQL built-in function that returns keys as a JSON array from a JSON member object at a given path, specified in the form of "$", "$.key", or "$[i]". For example:

SET @j = '{"a":null, "b":[1,[2,[3,4]]], "c":{"x":6,"y":[7,[8,9]]}}';

SELECT JSON_KEYS(@j), JSON_KEYS(@j, '$.b'), JSON_KEYS(@j, '$.c');
  -- +-----------------+----------------------+----------------------+
  -- | JSON_KEYS(@j)   | JSON_KEYS(@j, '$.b') | JSON_KEYS(@j, '$.c') |
  -- +-----------------+----------------------+----------------------+
  -- | ["a", "b", "c"] | NULL                 | ["x", "y"]           |
  -- +-----------------+----------------------+----------------------+

Reference information of the JSON_KEYS() function:

JSON_KEYS(json, path): array
  Returns keys as a JSON array from a JSON member object at a given path,
  path, specified in the form of "$", "$.key", or "$[i]".

Arguments, return value and availability:
  json: Required. The JSON value to be extracted from.
  path: Optional. The default is "$". The JSON path to the member object.
  array: Return value. The extracted keys.
  Available since MySQL 5.7.

⇒ JSON_LENGTH() - Calculating Length of JSON Value

⇐ JSON_INSERT() - Inserting Members to JSON Value

⇑ MySQL Functions on JSON Values

⇑⇑ MySQL Function References