drop table test_employee;

CREATE TABLE test_employee
( emp_id varchar2(50) NOT NULL, dept_id varchar2(50) NOT NULL,  emp_from_date date,  emp_to_date date);

INSERT INTO test_employee (emp_id, dept_id, emp_from_date,emp_to_date)
values ('ID_123','Dep_1',to_date('01-01-2015', 'dd-mm-yyyy'),to_date('31-05-2015', 'dd-mm-yyyy'));

INSERT INTO test_employee (emp_id, dept_id, emp_from_date,emp_to_date)
values ('ID_123','Dep_2',to_date('01-06-15', 'dd-mm-yyyy'),to_date('31-12-9999', 'dd-mm-yyyy'));

select * from test_employee;

Data stored in DB like below			
ID	Dept	From_date	To_date
ID_123	Dep_1	1-Jan-15	31-May-15
ID_123	Dep_2	1-Jun-15	31-Dec-99

Required output is:
Need to get results as below				
ID	Dept	From_date	To_date	        Comments
ID_123	Dep_1	1-Jan-15	31-May-15	
ID_123	Dep_1	1-Feb-15	31-May-15	
ID_123	Dep_1	1-Mar-15	31-May-15	
ID_123	Dep_1	1-Apr-15	31-May-15	
ID_123	Dep_1	1-May-15	31-May-15	
ID_123	Dep_2	1-Jun-15	31-Dec-99	Latest Record
ID_123	Dep_2	1-Jul-15	31-Dec-99	Till current Month
ID_123	Dep_2	1-Aug-15	31-Dec-99	Till current Month

WITH test_employees
        AS (SELECT 'ID_123' id,
                   'Dep_1' dept,
                   TO_DATE ('01-JAN-15', 'DD-MON-YY') from_date,
                   TO_DATE ('31-MAY-15', 'DD-MON-YY') TO_DATE
              FROM DUAL
            UNION ALL
            SELECT 'ID_123' id,
                   'Dep_2' dept,
                   TO_DATE ('01-JUN-15', 'DD-MON-YY') from_date,
                   TO_DATE ('31-DEC-99', 'DD-MON-YY') TO_DATE
              FROM DUAL)
    SELECT id,
           dept,
           ADD_MONTHS (from_date, LEVEL),
           TO_DATE
      FROM test_employees
CONNECT BY LEVEL <= MONTHS_BETWEEN (TO_DATE, from_date);