Postgres Trigger - Working with 2 different tables - postgresql

Question

I have 2 tables:
Books - isbn (PK), title, qty-in-stock
Orderlist - ordernum (PK), isbn, quantity
My goal is to insert a record into orderlist and then, if matching isbns, add the quantity to qty-in-stock
My function and trigger is not correct -
CREATE OR REPLACE FUNCTION books_upd() RETURNS trigger as $bookupd$
BEGIN
--IF THE ISBN MATCHES BETWEEN BOOKS AND ORDERLIST
-- ADD THE EXISTING QTY_IN_STOCK (BOOKS) TO QUANTITY (ORDERLIST)
QTY_IN_STOCK:=QTY_IN_STOCK+QUANTITY;
--END IF
RETURN NEW;
$bookupd$ LANGUAGE plpgsql;
CREATE TRIGGER books_upd
BEFORE INSERT OR UPDATE on orderlist
FOR EACH ROW
EXECUTE PROCEDURE books_upd();
Can anyone help?

Answer 1

The trigger function is quite complicated because of different cases of insert and update.
When a user updates the order, then the function should:
add to qty_in_stock the difference between new and old quantities if isbns are the same or
subtract old quantity from qty_in_stock of one book and add new quantity to qty_in_stock of another book when isbns are different.
Additionally, the function should reject changes if the given isbn does not exist in books.
CREATE OR REPLACE FUNCTION books_upd()
RETURNS trigger as $bookupd$
DECLARE
v_quantity int;
BEGIN
v_quantity = NEW.quantity;
IF TG_OP = 'UPDATE' THEN
IF OLD.isbn != NEW.isbn THEN
UPDATE books
SET qty_in_stock = qty_in_stock- OLD.quantity
WHERE isbn = OLD.isbn;
ELSE
v_quantity = NEW.quantity- OLD.quantity;
END IF;
END IF;
UPDATE books
SET qty_in_stock = qty_in_stock+ v_quantity
WHERE isbn = NEW.isbn;
IF NOT FOUND THEN
RAISE EXCEPTION 'Uknown isbn';
END IF;
RETURN NEW;
END;
$bookupd$ LANGUAGE plpgsql;
Read more about NEW, OLD and TG_OP.
The trigger function only for insert is really simple:
CREATE OR REPLACE FUNCTION books_upd()
RETURNS trigger as $bookupd$
BEGIN
UPDATE books
SET qty_in_stock = qty_in_stock+ NEW.quantity
WHERE isbn = NEW.isbn;
RETURN NEW;
END;
$bookupd$ LANGUAGE plpgsql;