Question

I have the following two tables in a MySql database:

Bookings
BookingID | ClientID | SeatID

SeatAvailability
SeatAvailabilityID | BookingID | ShowID | Available 

They are linked on SeatID/SeatAvailabilityID.
I’m trying to write a trigger which updates the SeatAvailability table each time a row is inserted in Bookings. The trigger should change SeatAvailability.Available to 0 and also enter the BookingID from Bookings into the BookingID field in SeatAvailability with the same SeatAvailabilityID.

I’ve written this trigger, MySql accepts it but gives an error when inserting
“ERROR 1054: Unknown column ‘cinemax.bookings.SeatID’ in ‘where clause'”.

DELIMITER $$

USE `cinemax`$$

CREATE
DEFINER=`root`@`localhost`
TRIGGER `cinemax`.`update_available`
AFTER INSERT ON `cinemax`.`bookings`
FOR EACH ROW
UPDATE cinemax.seatavailability
SET cinemax.seatavailability.Availabe=0, cinemax.seatavailability.BookingID=cinemax.bookings.BookingID
WHERE cinemax.bookings.SeatID=cinemax.seatavailability.SeatAvailabilityID$$

Answer 1

It’s a couple of months late, but I decided to give it a quick shot before handing in the overall assignment. In the meantime I switched to postgres as it seemed to offer more functionality (albeit not as user friendly). I first had to create a trigger function:

CREATE OR REPLACE FUNCTION updateseatavailable()
RETURNS trigger AS
$BODY$
BEGIN

            IF (TG_OP = 'INSERT') THEN
                UPDATE "SeatAvailability"
            SET "Available"='FALSE' AND "BookingID"=NEW."BookingID" WHERE "SeatAvailabilityID"=NEW."SeatID";

            ELSIF (TG_OP = 'DELETE') THEN
            UPDATE "SeatAvailability"
            SET "Available"='TRUE'  WHERE "SeatAvailabilityID"=OLD."SeatID";

            END IF;

            RETURN NEW;
        END;
    $BODY$
      LANGUAGE plpgsql VOLATILE

and then simply call the function/procedure from a trigger:

CREATE TRIGGER UpdateSeatAvailable
AFTER INSERT OR DELETE ON "Bookings"
FOR EACH ROW
EXECUTE PROCEDURE updateSeatAvailable();

I wasn’t able to get the BookingID in SeatAvailability to update for some reason (on Insert nothing happened and on Delete I got an error telling me Available cannot be null, even though I was changing the BookingID) so I omitted that in postgres,and implemented it with Java instead. It’s not the best way but still better than nothing.

I decided to post my solution just in case someone has a similar problem and stumbles upon this question.