Function error – postgresql - postgresql

Question

I created this function that checks that total cost of a flight has been calculated correctly. It should multiply the price of seats e.g. 400 by the number of seats a customer has asked for e.g. 2. If the customer inputs the total correctly e.g.800 it should return the query successfully. If they don't input the total correctly e.g. 1000 then it should raise the exception 'price has been calculated incorrectly'.
CREATE FUNCTION check_totalcost() RETURNS trigger AS $check_totalcost$
DECLARE seatprice int;
DECLARE noofseats int;
BEGIN
SELECT priceperseat INTO seatprice
FROM flight
WHERE flightid = flightid;
SELECT numseats INTO noofseats
FROM flightbooking
WHERE flightid = flightid;
IF(seatprice*noofseats != new.totalcost) THEN
RAISE EXCEPTION 'price has been calculated incorrectly';
END IF;
RETURN NEW;
END;
$check_totalcost$ LANGUAGE plpgsql;
/* This is used to run the function above */
CREATE TRIGGER insert_totalcost
BEFORE INSERT OR UPDATE ON flightbooking
FOR EACH ROW EXECUTE PROCEDURE check_totalcost();
When I input the total incorrectly, it raises the exception which is what I want but when I input the total correctly, it also raises the exception when it should return the query successfully.

Answer 1

The queries
SELECT priceperseat INTO seatprice
FROM flight
WHERE flightid = flightid;
SELECT numseats INTO noofseats
FROM flightbooking
WHERE flightid = flightid;
make little sense as the conditions are always true.
I guess you want to get flightid from inserted/updated record. Use the special record NEW:
WHERE flightid = NEW.flightid;
You have one more logical error in your function. The number of seats also should be got from the new record.
CREATE OR REPLACE FUNCTION check_totalcost()
RETURNS trigger AS $check_totalcost$
DECLARE seatprice int;
BEGIN
SELECT priceperseat INTO seatprice
FROM flight
WHERE flightid = new.flightid;
IF seatprice * new.numseats != new.totalcost THEN
RAISE EXCEPTION 'price has been calculated incorrectly';
END IF;
RETURN NEW;
END;
$check_totalcost$ LANGUAGE plpgsql;