string queryString = "select a.miesiace," +
        "(select ISNULL(sum(kwota), 0) from przychody c where DATEPART(month, (c.data_przychodu)) = a.miesiace" +
        "and DATEPART(YEAR, (c.data_przychodu)) = DATEPART(year, (getdate()))) -" +
        "(select ISNULL(sum(kwota), 0) from wydatki b where DATEPART(month, (b.data_zakupu)) = a.miesiace" +
        "and DATEPART(YEAR, (b.data_zakupu)) = DATEPART(year, (getdate())))";

            using (SqlConnection con1 = new SqlConnection(polaczenie_baza))
            {
                SqlCommand command = new SqlCommand(
            queryString, con1);
                con1.Open();
                SqlDataReader reader = command.ExecuteReader();
                try
                {
                    while (reader.Read())
                    {

                        MessageBox.Show(reader.ToString());
                    }
                }

                finally
                {
                    reader.Close();
                }
            }