1：本月第一天
select trunc(add_months(last_day(sysdate), -1) + 1) from dual
select trunc(sysdate,‘mm‘) from dual;
2：本月最后一天
select trunc(last_day(sysdate)) from dual;
3：上月的第一天和最后一天
select trunc(add_months(last_day(sysdate), -2) + 1),trunc(last_day(add_months(sysdate,-1))) from dual;
