Oracle 18c

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CREATE TABLE table_name (
  value CLOB CONSTRAINT ensure_json CHECK (value IS JSON)
);
INSERT INTO table_name ( value ) VALUES ( '{"a":"value1", "b":"value2", "c":"value3"}' );
1 rows affected

CREATE FUNCTION get_key(
  pos  IN PLS_INTEGER,
  json IN CLOB
) RETURN VARCHAR2 
AS
  doc_keys JSON_KEY_LIST;
BEGIN
  doc_keys := JSON_OBJECT_T.PARSE ( json ).GET_KEYS;
  RETURN doc_keys( pos );
END get_key;
/
SELECT * FROM USER_ERRORS;
SELECT get_key( j.pos, t.value ) AS key,
       j.value
FROM   table_name t
       CROSS APPLY JSON_TABLE(
         t.value,
         '$.*'
         COLUMNS (
           pos   FOR ORDINALITY,
           value PATH '$'
         )
       ) j;


KEY
VALUE


a
value1

b
value2

c
value3