string query = String.Format(@"SELECT pi.id 
                           FROM summary_order.price_item pi 
                           WHERE (pi.id = {0} or (pi.price_id = {1} and pi.provider_price_item_id = {2}))
                           ORDER BY pi.id limit 1", priceItem.Id, idPrice, priceItem.ProviderPriceItemId);
            using (var conn = new MySqlConnection("ххх"))
            using (var cmd = new MySqlCommand(query, conn))
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection.Open();
                int id = 0;
                MySqlDataReader reader = cmd.ExecuteReader();
                if (reader.Read())
                    id=reader.GetInt32("id");
                reader.Close();
                return id;
            }

ICriteria criteria = NHibernateStateLessSession.CreateCriteria(typeof(PriceItem)).
                AddOrder(NHibernate.Criterion.Order.Desc(PriceItem.Properties.Id)).
                SetFirstResult(0).
                SetMaxResults(1).
                Add(Restrictions.Or
                    (Restrictions.Eq(PriceItem.Properties.Id, priceItem.Id),
                              Restrictions.And(               
                                    Restrictions.Eq(String.Format("{0}.Id", PriceItem.Properties.Price), idPrice),
                                    Restrictions.Eq(PriceItem.Properties.ProviderPriceItemId, priceItem.ProviderPriceItemId))));
return criteria.UniqueResult<PriceItem>();