> insert into reservations(id,starttime,endtime) values
> (_id,_start,_end) where not exists (
> select id from reservations
> where (_start <= starttime and _end > starttime)
> or (_start > starttime and _end < endtime)
> or (_start < endtime and _end > starttime))
>
> I'm pretty sure the where statement is wrong.

Firebird does not support such INSERT statements (btw, which database
supports this?!).

Roman