A trigger in Oracle SQL - sql

Question

I am trying to create a trigger that after INSERT OR UPDATE OF columnX OR DELETE ON tableA
will increment or decrement by 1 columnY in tableB where tableB.key = tableA.key
For example: table product(droduct_id,product_rating) and table sale_line(sale_id, product_id,sale_qty)
Every time a greater quantity is entered into sale_line.sale_qty, product.product_rating is incremented by 1 and vice versa.
Thus, the greater is the quantity of a product in the sale_qty in the sale_line table, the greater is its rating in the product table
I have tried few variants found in different posts but none of them worked.
If I manage incrementing o decrementing the rating column, it goes for the whole table and not for the specific row.
How can I specify that changes in one table trigger changes in another table and all that happens for a certain product?
Now I know that the problem is my insufficient knowledge and some guys expressed the opinion that a trigger is the last thing to have, but this is why I want to understand it.
For the moment, this is what I have and it throws this error:
ORA-04079: invalid trigger specification
CREATE OR REPLACE TRIGGER t
AFTER INSERT OR UPDATE OF columnX OR DELETE
ON tableA
BEGIN
IF INSERTING AND :NEW.columnX > 2 THEN
UPDATE tableB
SET columnY = columnY + 1
WHERE columnY = 0;
ELSIF UPDATING AND :NEW.columnX < :OLD.columnX THEN
UPDATE tableB
SET columnY = columnY - 1
WHERE columnY <> 0;
ELSIF DELETING THEN
UPDATE tableB
SET columnY = 0
WHERE columnY <> 0;
END IF;
END;

Answer 1

Try it like this:
create or replace TRIGGER trg_aft_ins_upd_del_a_tbl
AFTER INSERT OR UPDATE OF COL_X OR DELETE
ON A_TBL
FOR EACH ROW
DECLARE
cnt NUMBER;
BEGIN
IF INSERTING AND :new.COL_X > 2 THEN
-- update if exist a row in B_TBL with ID = inserted ID - else insert new row in B_TBL
Select Count(*) INTO cnt From B_TBL Where ID = :new.ID;
IF cnt > 0 THEN
UPDATE B_TBL
SET COL_Y = COL_Y + 1
WHERE ID = :new.ID;
ELSE
INSERT INTO B_TBL (ID, COL_Y) VALUES(:new.ID, 1);
END IF;
-- if update decreased the value
ELSIF UPDATING AND :new.COL_X < :old.COL_X THEN
UPDATE B_TBL
SET COL_Y = COL_Y - 1
WHERE ID = :new.ID;
-- if update increased the value
ELSIF UPDATING AND :new.COL_X > :old.COL_X THEN
UPDATE B_TBL
SET COL_Y = COL_Y + 1
WHERE ID = :new.ID;
-- on delete set to 0
ELSIF DELETING THEN
UPDATE B_TBL
SET COL_Y = 0
WHERE ID = :old.ID;
END IF;
END;