Вычисление суммы

            decimal sum = 0;
            string connectionString = @"Data Source=.\SQLEXPRESS;Initial Catalog=Database1;Integrated Security=True";
                    SqlConnection conn = new SqlConnection(connectionString);
                    using (var cmd = conn.CreateCommand())
                    {
                        SqlDataReader myReader = null;
                        SqlCommand myCommand = new SqlCommand("SELECT SUM(CASE WHEN Trip.Date1 <= SpendByDays.Date AND Trip.Date2 >= SpendByDays.Date THEN SpendByDays.Price ELSE 0 END) AS k FROM SpendByDays, Trip WHERE Trip.ID_trip=SpendByDays.ID_trip", conn);
                        conn.Open();
                        myReader = myCommand.ExecuteReader();
                        while (myReader.Read())
                        {
                            sum = Convert.ToDecimal(myReader["k"].ToString());
                        }
                        myReader.Close();
                        conn.Close();
 
                        cmd.CommandText = "SELECT ListOfCity.NameOfCity, ListOfCity.NameOfCountry, ListOfTour.NameOfTour, Trip.Date1, Trip.Date2, Trip.Summa FROM ListOfCity, ListOfTour, Trip, SpendByDays WHERE ListOfCity.ID_city=Trip.ID_city AND ListOfTour.ID_tour=Trip.ID_tour AND Trip.ID_trip=SpendByDays.ID_trip AND Trip.Summa=@sum";
                        cmd.Parameters.Clear();
                        cmd.Parameters.AddWithValue("sum", sum);
                        conn.Open();
                        cmd.ExecuteNonQuery();
                        conn.Close();
                        dataTable3TableAdapter.Connection.Close();
                        dataTable3TableAdapter.Connection.Open();
                        dataTable3TableAdapter.Adapter.SelectCommand.CommandText = "SELECT ListOfCity.NameOfCity, ListOfCity.NameOfCountry, ListOfTour.NameOfTour, Trip.Date1, Trip.Date2, Trip.Summa," +
                            "Trip.ID_trip, SpendByDays.ID_trip, ListOfCity.ID_city, Trip.ID_city, ListOfTour.ID_tour, Trip.ID_tour FROM Trip, ListOfCity, SpendByDays, ListOfTour WHERE ListOfCity.ID_city=Trip.ID_city AND ListOfTour.ID_tour=Trip.ID_tour AND SpendByDays.ID_trip=Trip.ID_trip";
                        dataTable3TableAdapter.Adapter.SelectCommand.Parameters.Clear();                        
                        dataTable3TableAdapter.Adapter.SelectCommand.ExecuteReader();
                        dataTable3TableAdapter.Connection.Close();
                        database1DataSet.DataTable3.Clear();
                        dataTable3TableAdapter.Adapter.Fill(database1DataSet.DataTable3);                        
                        conn.Close();
                    }