Does your S.No column correspond directly to the empno (1, 1001; 2, 1002...)?

If not, you probably want an identity column which will automatically increment.  Otherwise, you could use substring to pull the last digit from empno.  By the way, I wouldn't name the new column "S.No", "S_No" would be better.  Stuff like spaces, periods, quote marks, etc can cause problems in object names.

Steve