问题：对于数字列中的值，计算其累计差。例如：计算DEPTNO 10中工资的累计差。

select a.empno,a.ename,a.sal,

(select case when a.empno=min(b.empno) then sum(b.sal)

else sum(-b.sal)

end

from emp b 

where b.empno<=a.empno

and b.deptno=a.deptno) as rnk

from emp a

where a.deptno=10;