Вложенный запрос вернул больше одного значения

USE [TourCompany]
GO
/****** Object:  StoredProcedure [dbo].[CountPrice]    Script Date: 21.11.2016 16:23:32 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[CountPrice](@id as INT) AS
BEGIN
    IF (SELECT Tour.id FROM Tour) = @id
    BEGIN
        DECLARE @startdate datetime;
        SET @startdate = (SELECT DISTINCT time_of_flight FROM Flight, Tour
                            WHERE (Flight.num = Tour.arrival_flight)); 
        DECLARE @enddate datetime;
        SET @enddate = (SELECT DISTINCT time_of_flight FROM Flight, Tour
                            WHERE (Flight.num = Tour.departure_flight));
        DECLARE @numofdays INT;
        SET @numofdays = DATEDIFF(day, @startdate, @enddate);
        DECLARE @onenight REAL;
        SET @onenight = (SELECT DISTINCT Accomodation.price FROM Accomodation, Tour
                            WHERE (Accomodation.id = Tour.accomodation));
        DECLARE @priceto INT;
        SET @priceto = (SELECT DISTINCT Flight.price FROM Flight, Tour
                                WHERE (Flight.num = Tour.departure_flight));
        DECLARE @pricefrom INT;
        SET @pricefrom = (SELECT DISTINCT Flight.price FROM Flight, Tour
                                WHERE (Flight.num = Tour.departure_flight));
        UPDATE Tour 
        SET Tour.price = 1.5*(@numofdays*@onenight+@pricefrom+@priceto);
    END
END

USE TourCompany;
GO
EXEC dbo.CountPrice 1;

SET @startdate = (SELECT DISTINCT time_of_flight FROM Flight, Tour
                            WHERE (Flight.num = Tour.arrival_flight));