Problem With Custom Reservation System, Date Querying

Hi, I'm having issues trying to get a query working the way I want, it maybe
that i'm overly complicating things though.

What I have done so far is have 2 seperate tables one holding details about
the item to be booked out with an ID the second linking to the Item via the
ID and also having the startdate and the enddate of the booking, thus an item
will have multiple rows in the bookings table for multiple bookings.

What I want to have is a "quick" booking method where a user enters the
startdate they would like and the enddate, a drop down is then filtered (via
a query) returning only the items that are avalible.

The issue i'm having is that because my bookings have multiple rows for each
item, for what maybe true in the rules for an item in 1 row maybe false
alittle later - i which case the returned data i am getting is incorrect!

Hopefully I have made sense, and maybe someone can help?

my querry for the filter so far: -

SELECT DISTINCT DeviceDetails.Device_ID, DeviceDetails.Device_Name
FROM DeviceDetails LEFT OUTER JOIN
BookingDetails ON BookingDetails.Device_ID =
DeviceDetails.Device_ID
WHERE (BookingDetails.Bookout_Date IS NULL) OR
(BookingDetails.Bookout_Date >= GETDATE()) AND
(@Dateout <= BookingDetails.Bookin_Date) AND (BookingDetails.Bookout_Date >=
@Datein) OR
(BookingDetails.Bookout_Date >= GETDATE()) AND
(@Dateout >= BookingDetails.Bookin_Date) AND (BookingDetails.Bookout_Date <=
@Datein)