How to use Trigger with joining 2 tables? - oracle

Question

I have 2 tables one is Schedule_table :
SID NOT NULL NUMBER(38)
FNUMBER VARCHAR2(20)
DEPARTURE_TIME TIMESTAMP(6) WITH TIME ZONE
ARRIVAL_TIME TIMESTAMP(6) WITH TIME ZONE
PRICE NUMBER
The second table is Flight_table
FNUMBER NOT NULL VARCHAR2(20)
DEPARTURE_APCODE CHAR(3)
ARRIVAL_APCODE CHAR(3)
Fnumber is PK in Flight_table and FK in Schedule_table.
I want the trigger to fire when the PRICE gets changed in Schedule_table
Also, to print a message of (Fnumber, ARRIVAL_APCODE, DEPARTURE_TIME ,ARRIVAL_APCODE).
I wrote this Code, it did not work.
create or replace trigger schedule_trigger after update on schedule
for each row when (new. price <> old. price)
begin
dbms_output.put_line( 'the flight number ' || :new.fnumber|| DEPARTURE_APCODE ||DEPARTURE_TIME|| ' has changed to '||:new.price ||' From'||:old.price);
end;

Answer 1

You are referring to the columns of Flight table - DEPARTURE_APCODE and DEPARTURE_TIME directly inside trigger which is not possible.You need to use a select
to fetch those values.
CREATE OR REPLACE TRIGGER schedule_trigger AFTER
UPDATE ON schedule
FOR EACH ROW
WHEN ( new.price <> old.price )
DECLARE
v_departure_apcode flight.departure_apcode%TYPE;
v_arrival_apcode flight.arrival_apcode%TYPE;
BEGIN
SELECT
departure_apcode,
arrival_apcode
INTO
v_departure_apcode,v_arrival_apcode
FROM
flight
WHERE
fnumber =:new.fnumber;
dbms_output.put_line('The flight number '
||:new.fnumber
|| v_departure_apcode
|| v_arrival_apcode
|| ' has changed to '
||:new.price
|| ' From '
||:old.price);
END;
/
And note that using DBMS_OUTPUT.PUT_LINE() inside a trigger is of no use as SQL developer sometimes does not display your output. Better create a log table and insert the records into it in your trigger.
You may require a commit statement for the output to be displayed.
SET SERVEROUTPUT ON;
UPDATE Schedule SET PRICE = 2502.00 where fnumber = 'F2004' ;
commit;
Commit complete.
The flight number F2004AB CD has changed to 2502 From 2501