Oracle Triggers Update at Another Table - database

Question

I am trying to create a trigger in Oracle. I know sql but i have never created trigger before. I have this code:
create or replace trigger "PASSENGER_BOOKING_T1"
AFTER
insert on "PASSENGER_BOOKING"
for each row
begin
IF (:NEW.CLASS_TYPE == 'ECO')
SELECT F.AVL_SEATS_ECOCLASS,F.FLIGHT_ID INTO SEAT, FLIGHT_INFO
FROM BOOKING B, JOURNEY_FLIGHT J, FLIGHT F
WHERE B.JOURNEY_ID = J.JOURNEY_ID and F.FLIGHT_ID = J.FLIGHT_ID;
UPDATE FLIGHT
SET AVL_SEATS_ECOCLASS = (SEAT-1)
WHERE FLIGHT_ID = FLIGHT_INFO;
END IF;
end;​
This trigger fires when there is an insert in Passenger_Booking table. And seating capacity is reduced by one (which is at different table).
Select query should be alright but there is something wrong in somewhere.
Could anyone suggest anything?
I changed the body part to this but still having issues:
UPDATE FLIGHT
SET AVL_SEATS_ECOCLASS =
(SELECT F.AVL_SEATS_ECOCLASS FROM BOOKING B, JOURNEY_FLIGHT J, FLIGHT F WHERE B.JOURNEY_ID = J.JOURNEY_ID and F.FLIGHT_ID = J.FLIGHT_ID;
);

Answer 1

An IF statement needs a THEN
In PL/SQL, you use an = to test for equality, not ==
You need to declare the variables that you are selecting into
When I do those three things, I get something like this
create or replace trigger PASSENGER_BOOKING_T1
AFTER insert on PASSENGER_BOOKING
for each row
declare
l_seat flight.seat%type;
l_flight_id flight.flight_id%type;
begin
IF (:NEW.CLASS_TYPE = 'ECO')
THEN
SELECT F.AVL_SEATS_ECOCLASS,F.FLIGHT_ID
INTO l_seat, l_flight_id
FROM BOOKING B,
JOURNEY_FLIGHT J,
FLIGHT F
WHERE B.JOURNEY_ID = J.JOURNEY_ID
and F.FLIGHT_ID = J.FLIGHT_ID;
UPDATE FLIGHT
SET AVL_SEATS_ECOCLASS = (l_seat-1)
WHERE FLIGHT_ID = l_flight_id;
END IF;
end;​
Beyond those syntax errors, I would be shocked if the SELECT INTO statement was correct. A SELECT INTO must return exactly 1 row. Your query should almost certainly return multiple rows since there are no predicates that would restrict the query to a particular flight or a particular booking. Presumably, you want to join to one or more columns in the PASSENGER_BOOKING table.
Additionally, if this is something other than a homework assignment, make sure you understand that this sort of trigger does not work correctly in a multi-user environment.

Answer 2

just a wild guess
edit as Justin pointed out (thanks Justin) equality check
create or replace trigger "PASSENGER_BOOKING_T1"
AFTER
insert on "PASSENGER_BOOKING"
for each row
declare
v_flight_id FLIGHT.FLIGHT_ID%TYPE;
begin
IF (:NEW.CLASS_TYPE = 'ECO') THEN
SELECT F.ID into v_flight_id
FROM BOOKING B, JOURNEY_FLIGHT J, FLIGHT F
WHERE B.ID = :NEW.BOOKING_ID -- note that I've made this up
AND B.JOURNEY_ID = J.JOURNEY_ID AND F.FLIGHT_ID = J.FLIGHT_ID;
UPDATE FLIGHT
SET AVL_SEATS_ECOCLASS = (SEAT-1)
WHERE ID = v_flight_id;
END IF;
end;​