sql oracle - constraint on 2 columns from different tables - sql

Question

I have designed a ticket system booking for flights. I want to add a constraint such that the number of tickets you can insert to be less than number of seats from a flight plane.
Let's say I inserted a flight with a plane with 10 seats. I can insert only 10 tickets for that particular flight. Otherwise, an error message should appear.
I tried to make a trigger using the count function on flight number.
CREATE OR REPLACE TRIGGER trg_ticket_BRIU
BEFORE INSERT OR UPDATE ON Ticket
FOR EACH ROW
DECLARE
l_numberofseats flight.numberofseats%type;
BEGIN
select numberofseats into l_numberofseats
from flight
where flightnumber=:new.flightnumber;
IF :new.count(flightnumber) > l_numberofseats
THEN
raise_application_error(-2000, 'Not enough seats');
END IF;
END;
but I get this error
Trigger TRG_TICKET_BRIU compiled
LINE/COL ERROR
--------- -------------------------------------------------------------
8/5 PLS-00049: bad bind variable 'NEW.COUNT'
Errors: check compiler log

Answer 1

Personally, I would add an AIRCRAFT and a SEAT table:
CREATE TABLE aircraft (
id NUMBER
GENERATED ALWAYS AS IDENTITY
CONSTRAINT aircraft__id__pk PRIMARY KEY,
tail_number VARCHAR2(6)
NOT NULL
CONSTRAINT aircraft__tn__u UNIQUE
CONSTRAINT aircraft__tn_chk CHECK(
REGEXP_LIKE(
tail_number,
'[A-Z]\d{1,5}|[A-Z]\d{1,4}[A-Z]|[A-Z]\d{1,3}[A-Z]{2}'
)
),
manufacturer VARCHAR2(20)
NOT NULL,
model VARCHAR2(20)
NOT NULL,
airline_id CONSTRAINT aircraft__aid__fk REFERENCES airline(airline_id)
NOT NULL
);
CREATE TABLE seat (
id NUMBER
GENERATED ALWAYS AS IDENTITY
CONSTRAINT seat__id__pk PRIMARY KEY,
aircraft_id CONSTRAINT seat__aid__fk REFERENCES aircraft(id)
NOT NULL,
seat_row VARCHAR2(3)
NOT NULL,
seat_column NUMBER
NOT NULL,
CONSTRAINT seat__aid_r_c__u UNIQUE (aircraft_id, seat_row, seat_column)
);
Then your flight table would reference the aircraft:
CREATE TABLE flight (
id NUMBER
GENERATED ALWAYS AS IDENTITY
CONSTRAINT flight__id__pk PRIMARY KEY,
aircraft_id CONSTRAINT flight__aid__fk REFERENCES aircraft(id)
NOT NULL
-- ...
);
And the ticket would reference a flight and a seat:
CREATE TABLE ticket (
id NUMBER
GENERATED ALWAYS AS IDENTITY
CONSTRAINT ticket__id__pk PRIMARY KEY,
flight_id CONSTRAINT ticket__fid__fk REFERENCES flight(id)
NOT NULL,
seat_id CONSTRAINT ticket__sid__fk REFERENCES seat(id)
NOT NULL,
-- ...
CONSTRAINT ticket__fid_sid__u UNIQUE (flight_id, seat_id)
);
Then you can never sell a seat that does not exist on an aircraft and do not need to count the maximum number of tickets and compare it to seats (and the seat has added attributes like its location on the plane that can be displayed on the ticket).
All you need then is to ensure the referential consistency that, for a ticket, the flight and the seat are on the same aircraft; which can be done with a trigger:
CREATE TRIGGER ticket_check_seat_on_flight
BEFORE INSERT OR UPDATE ON ticket
FOR EACH ROW
DECLARE
is_valid NUMBER(1);
BEGIN
SELECT 1
INTO is_valid
FROM flight f
INNER JOIN seat s
ON (f.aircraft_id = s.aircraft_id)
WHERE f.id = :NEW.flight_id
AND s.id = :NEW.seat_id;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RAISE_APPLICATION_ERROR(
-20000,
'Flight and seat are on different aircraft.'
);
END;
/
db<>fiddle here

Answer 2

You can use an AFTER STATEMENT trigger:
CREATE TRIGGER ticket__check_number_of_seats
AFTER INSERT OR UPDATE OR DELETE ON ticket
DECLARE
is_invalid NUMBER(1,0);
BEGIN
SELECT 1
INTO is_invalid
FROM flight f
INNER JOIN (
SELECT flight_id,
COUNT(*) AS tickets_sold
FROM ticket
GROUP BY flight_id
) t
ON f.id = t.flight_id
WHERE t.tickets_sold > f.number_of_seats;
RAISE_APPLICATION_ERROR(
-20000,
'Too many tickets sold for flight.'
);
EXCEPTION
WHEN NO_DATA_FOUND THEN
NULL;
END;
/
It could be made more efficient by using a compound trigger to collate, for each row, the flight_id values into a collection and then, after the statement, only checking the number of tickets for those flights; however, I'll leave that extension as an exercise for the OP.
db<>fiddle here

Answer 3

As others indicated there is no :new.count column. This is because :new (and :old) create a pseudo-row containing exactly the same columns as the table definition. Further you will get a Mutating exception as what you need to count in the flight_number from tickets. However, since that is the table causing he trigger to fire you cannot reference it. So what to do: create a compound trigger, and a supporting Type (nested table). Within it use the after row section to capture the flight_numbers processed. Then in the after statement section you can select count of tickets for each flight. If that count > 0 then raise your exception. ( see Demo )
create type flight_tickets_ntt
is table of integer;
create or replace trigger trg_ticket_ciu
for update or insert on tickets
compound trigger
l_flights flight_tickets_ntt := flight_tickets_ntt();
after each row is
begin
if :new.flight_number not member of l_flights then
l_flights.extend ;
l_flights(l_flights.count) := :new.flight_number;
end if;
end after each row;
after statement is
l_flight_cnt flight.flight_number%type;
begin
select count(*)
into l_flight_cnt
from flight f
where f.number_of_seats <
( select count(*)
from tickets t
where t.flight_number in
( select *
from table (l_flights)
)
);
if l_flight_cnt > 0 then
raise_application_error(-20000, 'Not enough seats');
end if;
end after statement;
end trg_ticket_ciu;
There remains a you need to handle: What happens if an update changes the flight number or perhaps (missing column) the data of the flight.