Question

I am trying to update a table according to this trigger:

CREATE TRIGGER alert 
AFTER UPDATE ON cars
FOR EACH ROW
EXECUTE PROCEDURE update_cars();

Trigger Function :

CREATE FUNCTION update_cars()
RETURNS 'TRIGGER' 
AS $BODY$
BEGIN 
IF (TG_OP = 'UPDATE') THEN
UPDATE hello_cars SET status = new.status 
WHERE OLD.ID = NEW.ID;
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;

The trigger works fine. When the cars table is updated, the hello_cars table is updated but the status column in each row is updated and contains same new status! It must be updated according to a car ID.
I think my problem is in condition: WHERE OLD.ID = NEW.ID; but I can’t tell what’s wrong.

Thanks in advance.

Answer 1

OLD and NEW are aliases to the rows which fired the trigger. So when you execute a statement like

UPDATE cars SET status='xyz' WHERE cars.id = 42;

then the trigger function will execute

UPDATE hello_cars SET status='xyz' WHERE 42 = 42

The part 42=42 is always true. So each row in hello_cars is updated.

You really want something like

 [...]WHERE hello_cars.id = OLD.ID

or a little shorter

 [...]WHERE id = OLD.ID

But you also need to think about what happens, if the initial update changes cars.id. In this case OLD.ID is not equal NEW.ID. What should happen in the table hello_cars in this case? But that’s another question.