PostgreSQL how to write trigger - postgresql

Question

Say I have a following table like this:
http://i.stack.imgur.com/qU3gh.png
where sid stands for sailor ID, and bid stands for boat ID.
I want to write a trigger and a function to detect whether the new input is invalid. Look at the table, the 3rd record shouldn't be there because the renting time has overlap with the former one, one boat can't be rented before it is returned. That is to say, I want a trigger and a function to stop input like the 3rd one, when someone tries to give a 3rd input, it will just stop you from doing so.
Currently I wrote the following code, but as I am new to this, I am really not sure whether it is correct:
CREATE FUNCTION update() RETURNS TRIGGER AS $logupdate$
DECLARE
judge boolean;
BEGIN
judge := EXECUTE ( 'SELECT starttime,endtime,NEW.starttime,NEW.endtime FROM reserves WHERE bid = NEW.bid AND startdate = NEW.startdate AND (starttime,endtime) overlaps(NEW.starttime,NEW.endtime) IS NULL');
IF judge = f THEN RAISE EXCEPTION 'failed due to some reasons';
END IF;
RETURN NEW;
END;
$logupdate$ LANGUAGE plpgsql;
CREATE TRIGGER logupdate
before UPDATE ON reserves
FOR EACH ROW
EXECUTE PROCEDURE update();
How can I correct this?

Answer 1

consider using a timestamp field instead of seperate date and time fields for simplicity. you can easily format a timestamp into either a date or time.
update and insert triggers must return NEW on success, or FALSE/NULL
in order to enforce a constraint with a trigger you must run a BEFORE UPDATE trigger, otherwise the row's already added.
CREATE TRIGGER logupdate
BEFORE UPDATE ON reserves
FOR EACH ROW EXECUTE PROCEDURE update();
CREATE FUNCTION update() RETURNS TRIGGER AS $$
BEGIN
minstart := EXECUTE ( 'SELECT MAX(endtime) FROM reserves WHERE bid = NEW.bid AND startdate = NEW.startdate' ) ;
IF minstart < NEW.starttime THEN RETURN FALSE;
RETURN NEW;
END
$$