SQL trigger to decrement another table - sql

Question

I've created a Database for a DVD store which contains a Film table to hold info about films + copies available, and a sale table. I've created sequences for the Film and Sale table, to auto-increment the Primary keys FILM_ID and SALE_TRANSACTION_REF respectively, using the following code.
CREATE SEQUENCE Film_Seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
NOCACHE;
CREATE OR REPLACE TRIGGER New_Film
BEFORE INSERT ON FILM
FOR EACH ROW
BEGIN
SELECT Film_Seq.NEXTVAL
INTO :new.FILM_ID
FROM dual;
END;
/
This seems to work handy enough, but when I try to use a trigger to decrement the COPIES_AVAILABLE attribute of the film in Film after a sale with the following code, I get an error:
CREATE OR REPLACE TRIGGER After_Sale
AFTER INSERT ON SALE
DECLARE
ID NUMBER(38) := Sale_Seq.CURRVAL;
BEGIN
UPDATE FILM
SET COPIES_AVAILABLE = COPIES_AVAILABLE - 1
FROM FILM, SALE
WHERE FILM.FILM_ID = SALE.FILM_ID
AND (SALE.SALE_TRANSACTION_REF = ID);
END;
/
I actually get two errors, the first one relates to the declare statement, which is
4/2 PL/SQL: SQL Statement ignored
And the second one has to do with the UPDATE statement and it is
6/2 PL/SQL: ORA-00933: SQL command not properly ended
I've wrecked my brains trying to construct this statement differently, and have searched the documentation but can't seem to find the answer, and help would be greatly appreciated.
Also, looking over my question it may not be obvious but I have already created a sale_seq.
Sorry, it may not be evident what I am trying to achieve here. Whenever a Film is sold and added to the sale table, I want to automatically decrement the Copie_Available for the corresponding film in the film table, so if I left out the AND AND (SALE.SALE_TRANSACTION_REF = Sale_Seq.CURRVAL) clause the trigger will complie grand, but will decrement all copies available for all films in the film table, which is incorrect.

Answer 1

try it like this:
CREATE OR REPLACE TRIGGER After_Sale
AFTER INSERT ON SALE
FOR EACH ROW
BEGIN
UPDATE FILM
SET COPIES_AVAILABLE = COPIES_AVAILABLE - 1
WHERE FILM.FILM_ID = :NEW.FILM_ID
END;

Answer 2

Why save COPIES_AVAILABLE to your database. Why wouldn't you detract for instance [all copies in stock] - [all sold copies] if you want to know available copies.