Return results of a sql query as JSON in oracle 12c

12cR2 (available in the Oracle Cloud) supports this natively.

SQL> select JSON_ARRAY(EMPLOYEE_ID, FIRST_NAME,LAST_NAME) from HR.EMPLOYEES;

JSON_ARRAY(EMPLOYEE_ID,FIRST_NAME,LAST_NAME)
--------------------------------------------------------------------------------
[100,"Steven","King"]
[101,"Neena","Kochhar"]

or

SQL> select JSON_OBJECT('ID' is EMPLOYEE_ID , 'FirstName' is FIRST_NAME,'LastName' is LAST_NAME) from HR.EMPLOYEES;

JSON_OBJECT('ID'ISEMPLOYEE_ID,'FIRSTNAME'ISFIRST_NAME,'LASTNAME'ISLAST_NAME)
----------------------------------------------------------------------------
{"ID":100,"FirstName":"Steven","LastName":"King"}
{"ID":101,"FirstName":"Neena","LastName":"Kochhar"}