oracle 日期、月份处理

 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;