SQL - Date get ' first day month-1 / last day month-1 / first day current month '

select 
  ADD_MONTHS(LAST_DAY(SYSDATE),-2)+1 "FIRST DAY, MONTH-1", 
  LAST_DAY(ADD_MONTHS(SYSDATE,-1)) "LAST DAY MONTH-1",
  ADD_MONTHS(LAST_DAY(SYSDATE),-1)+1 "FIRST DAY CURRENT MONTH"
from dual;