sql中日期相减

sql中两个日期相减
1、相差天数
select trunc(sysdate,'yyyy')-to_date('2009-01-04','yyyy-mm-dd') from dual;
2、相差月数
select  months_between(trunc(sysdate,'mm'),to_date('2009-01','yyyy-mm')) from dual; 
3、相差年数
select trunc((months_between(trunc(sysdate,'dd'),to_date('2009-01-01','yyyy-mm-dd')))/12) from dual;