Oracle取得本周，本月，本季，本年的第一天和最后一天

--本周
select trunc(sysdate,'d')+1 from dual;
select trunc(sysdate,'d')+7 from dual; 
--本月
select trunc(sysdate,'mm') from dual;
select last_day(trunc(sysdate)) from dual;
--本季
select trunc(sysdate,'Q') from dual;
select add_months(trunc(sysdate,'Q'),3)-1 from dual;
--本年
select trunc(sysdate,'yyyy') from dual;
select add_months(trunc(sysdate,'yyyy'),12)-1 from dual;