SQL & PL/SQL

Hello All,
I have a scenario where I'm using sum() function.

This works fine

DECLARE
   v_sal emp.sal%type;
BEGIN
   SELECT sal INTO v_sal FROM emp WHERE empno = 50;
EXCEPTION
WHEN NO_DATA_FOUND THEN
...
END;

But I can't use NO_DATA_FOUND in the case that I've mentioned below. Is there a way that I can catch here.
SELECT sum(sal) INTO v_sal FROM emp WHERE empno = 50;

Thx
Shank.