Get the start and end of each month with the help of Connect by clause

SELECT     ADD_MONTHS (TRUNC (SYSDATE, 'MM'), LEVEL - 500) st_end,

           ADD_MONTHS (ADD_MONTHS (TRUNC (SYSDATE, 'MM') - 1, 1),

                       LEVEL - 500

                      ) en_date

      FROM DUAL

CONNECT BY LEVEL < 1000