in some RDBMS (like PostgreSQL or SQL Server 2012 or Oracle) it's possible to use window function:
select
Emp, Sal, sum(Sal) over(order by Emp) as Cum_Sal
from employees
if it's not possible, you can use self-join
select
e1.Emp, e1.Sal, sum(e2.Sal) as Cum_Sal
from employees e1
inner join employees e2 on e2.Emp <= e1.Emp
group by e1.Emp, e1.Sal
Note that in both cases you have to define order for rolling sum (in queries above, it's ordered by Emp
column).