oracle last_value的问题解决一下

select deptno,sal,last_value(sal)over(partition by deptno order by sal) sum from emp where deptno=10;为什么部门10的sum的三行结果都是不一样的啊不是取得10部门sal排序后的最后一个值吗

select job,deptno,last_value(deptno)over(partition by job order by deptno) sum from emp where job='CLERK'

select job,deptno,last_value(deptno) over( partition by job order by deptno rows between unbounded preceding and unbounded following) sum from emp where job='CLERK'