SQL & PL/SQL

I HAVE A EMP TABLE LIKE BELOW... A SINGLE EMPLOYEE CAN HAVE MULTIPLE ASSIGNMENTS BUT CAN HAVE ONLY ONE PRIMARY ASSIGNMENT AT TIME. I WANT TO FIND THE EMPLOYEES
whose has new primary assignment(A change in assignment id will indicates change in assignment).

      
      SELECT 10 EMP_ID  , '100'EMP_NO ,  1  ASSINMENT_ID, '01-JAN-2011' ASSIGNMENT_START_DATE , '01-FEB-2011' ASSIGNMENT_END_DATE , 'Y' PRIMAY_ASSIGNMENT_FLAG FROM DUAL
      UNION ALL
      SELECT 10 EMP_ID  , '100'EMP_NO ,  2  ASSINMENT_ID, '02-FEB-2011' ASSIGNMENT_START_DATE , '31-DEC-4712' ASSIGNMENT_END_DATE , 'Y' PRIMAY_ASSIGNMENT_FLAG FROM DUAL
      UNION ALL
      SELECT 11 EMP_ID  , '100'EMP_NO ,  3  ASSINMENT_ID, '01-JAN-2011' ASSIGNMENT_START_DATE , '01-FEB-2011' ASSIGNMENT_END_DATE , 'Y' PRIMAY_ASSIGNMENT_FLAG FROM DUAL
      UNION ALL
      SELECT 11 EMP_ID  , '100'EMP_NO ,  3  ASSINMENT_ID, '02-FEB-2011' ASSIGNMENT_START_DATE , '31-MAR-2012' ASSIGNMENT_END_DATE , 'Y' PRIMAY_ASSIGNMENT_FLAG FROM DUAL
      UNION ALL
      SELECT 11 EMP_ID  , '100'EMP_NO ,  4  ASSINMENT_ID, '01-APR-2012' ASSIGNMENT_START_DATE , '31-DEC-4712' ASSIGNMENT_END_DATE , 'Y' PRIMAY_ASSIGNMENT_FLAG FROM DUAL
      
      

I need the outup like this .. single record for single employee
emp_id, emp_no , current_assignment_start_date, previous_assignment_start_date, current_assignment_end_date , previous_assignment_end_date

Thanks
Vinoth