i have the following method, at the moment it's return the whole sql string. How would i execute the following.

            using (ITransaction transaction = session.BeginTransaction())
            {
                string sql =
                    string.Format(
                        @"DECLARE @Cost money
                        SET @Cost = -1
                        select @Cost = MAX(Cost) from item_costings
                         where Item_ID = {0}
                        and {1} >= Qty1 and {1} <= Qty2
                        RETURN (@Cost)",
                        itemId, quantity);

               string mystring = session
                    .CreateSQLQuery(sql)
                    .ToString();

                transaction.Commit();
                return mystring;
            }

// EDIT

here is the final version using criteria

 using (ISession session = NHibernateHelper.OpenSession())
        {
           decimal cost = session
                .CreateCriteria(typeof (ItemCosting))
                .SetProjection(Projections.Max("Cost"))
                .Add(Restrictions.Eq("ItemId", itemId))
                .Add(Restrictions.Le("Qty1", quantity))
                .Add(Restrictions.Ge("Qty2", quantity))
                .UniqueResult<decimal>();
            return cost;
        }