Oracle trigger to update an entity in different table - oracle

Question

So this is from a uni coursework and is my first time working with Oracle (and using triggers). We are supposed to be creating a database for an airlines.
Part of the database is
CREATE TABLE FLIGHT_BOOKING (
BOOKING_ID NUMBER(11) PRIMARY KEY,
BOOKING_TIME DATE NOT NULL,
EMPLOYEE_ID NUMBER(11) NOT NULL,
FLIGHT_ID NUMBER(11) NOT NULL,
TOTAL_COST NUMBER(4,2) NOT NULL
);
CREATE TABLE FLIGHT (
FLIGHT_ID NUMBER(11) PRIMARY KEY,
PLANE_ID NUMBER(11) NOT NULL,
START_ID NUMBER(11) NOT NULL,
DESTINATION_ID NUMBER(11) NOT NULL,
TRANSIT_ID NUMBER(11),
DEPARTURE_TIME DATE NOT NULL,
ARRIVAL_TIME DATE NOT NULL,
NUM_BOOKED NUMBER (4) NOT NULL
);
CREATE TABLE PASSENGER (
PASSENGER_ID NUMBER(11) PRIMARY KEY,
FIRST_NAME VARCHAR2(20) NOT NULL,
MIDDLE_NAME VARCHAR2(20) NULL,
LAST_NAME VARCHAR2(20) NOT NULL,
TELEPHONE NUMBER(11) NOT NULL,
BOOKING_ID NUMBER(11) NOT NULL
);
So what I want to do is create a trigger such that every time a new passenger is added to the PASSENGER table, the trigger finds the corresponding FLIGHT_ID from the FLIGHT_BOOKING table and increments NUM_BOOKED for the corresponding flight in the FLIGHT table.
I have tried going through the oracle documentation, but i could not find anything that describes a situation where two or more tables are concerned.
Any help would be really appreciated!

Answer 1

you can do it like this:
CREATE OR REPLACE TRIGGER update_flight_booking_info
AFTER INSERT ON PASSENGER
FOR EACH ROW
DECLARE
v_flight_id number;
v_booking_id number;
BEGIN
v_booking_id := :new.booking_id ;
select flight_id into v_flight_id
from flight_booking
where booking_id = v_booking_id;
update flight
set NUM_BOOKED = NUM_BOOKED + 1
where flight_id = v_flight_id;
END;
HTH.

Answer 2

I'd rather not store that number, and calculate it as needed, but okay, it is just course material. :)
When you create a trigger, inside it you can put all kinds of code, including update statements.
So you can write a trigger like this:
create or replace trigger TIDB_BOOKING
before insert or delete
for each row
declare
V_Increment int;
begin
-- Inc or dec, depending on insert or update.
-- Hasn't a booking got a number of seats?
-- Also, can bookings be updated/moved to other flights?
-- These problems aren't yet taken into account in this code.
V_Increment := 1;
if deleting then
V_Increment := -1;
update FLIGHT f
set f.NUM_BOOKED = f.NUM_BOOKED + V_Increment
where f.FLIGHT_ID = nvl(:new.FLIGHT_ID, :old.FLIGHT_ID);
end;