Raj,
I believe that the formula has to be enclosed in parentheses because it's doing a subquery.
Also, it probably makes more sense to declare the data type to be some numeric type (BigDecimal?) because it's a sum, although I doubt that's causing the problem.
So, try something like this (untested):
Code:
<property name="empderived" type="double" formula="(select sum(e.sal+e.comm) from Emp e where e.ename = ename)" />
I used double because that's what your mapping of sal used, but you really want to be using BigDecimal for money fields.
Good luck,
Oscar