DataReader返回null结果，而在PgAdmin中执行相同的查询会返回结果

Question

我正在尝试执行查询：

select d1.rtime, d2.rtime,
   (d2.time_on_auto - d1.time_on_auto) as delta1,
   (d2.time_on_zal - d1.time_on_zal) as delta2
from (select *
  from db_reso3f_operation_month
  where rtime = '2013-02-01' AND gateway_id = 11226 AND f2 = 2 AND f1 = 4
 ) d1 cross join
 (select *
  from db_reso3f_operation_month
  where rtime = '2013-03-01' AND gateway_id = 11226 AND f2 = 2 AND f1 = 4
 ) d2

当我在DB编辑器中执行它时（我正在使用PgAdmin），它会返回正确的结果。但是当我尝试使用ExecuteReader（）执行它时，没有返回任何结果。它的代码如下所示：

using (NpgsqlCommand dbCommand = DBConnection.CreateCommand())
{
    string sql1 = string.Format(@"select d1.rtime, d2.rtime," + 
                  "(d2.time_on_auto - d1.time_on_auto) as delta1," + 
                  "(d2.time_on_zal - d1.time_on_zal) as delta2" +
                  " from (select *" +
                  " from db_reso3f_operation_month" +
                  " where rtime = '2013-02-01' AND gateway_id = {0} AND f2 = {1} AND f1 = {2}" +
                  ") d1 cross join" +
                  "(select *" +
                  "from db_reso3f_operation_month" +
                  " where rtime = '2013-03-01' AND gateway_id = {0} AND f2 = {1} AND f1 = {2}" +
                  ") d2",
                  device.GatewayId,
                  device.F2,
                  device.ReferenceCircuit);

    dbCommand.CommandText = sql1;
    using (IDataReader dataReader = dbCommand.ExecuteReader())
          {
           while (dataReader.Read())
              {
             switchedOnTime += (int)dataReader["delta1"];
             switchedOnTime += (int)dataReader["delta2"];
              }
           }

我现在不知道问题是什么，以及找不到有关npgsql是否支持此类查询的任何信息。