text: -- Get the first of the month out of the current month
  SELECT TRUNC(LAST_DAY(sysdate),'MM') FROM DUAL;
  
  -- To get the last day just use the last day function 
  SELECT LAST_DAY(sysdate),'MM' FROM DUAL;
  
  --To get the first of the month from the previous month just use add_months to get the previous month 
  SELECT ADD_MONTHS(TRUNC(LAST_DAY(sysdate),'MM'),-1) FROM DUAL;