Need help on a database model design
Your question is not very clear on this point but I'm assuming you want to find overlaps with a proposed, new booking.

Let the values for the proposed booking be held in the variables @StartDate, @EndDate and @RoomID.  The SQL will be

select
    RoomID,
    StartDate,
    EndDate
from TheBookingTable as t
where t.RoomID =  @RoomID
and T.StartDate &lt;= @EndDate
and T.EndDate &gt;= @StartDate;


This query will return all current bookings which overlap your proposed booking, or an empty set if none exist.