Hvis du vil gå med en triggertilgang, skal du bruge to triggere (til indsættelse og sletning)
CREATE TRIGGER tg_ai_orders
AFTER INSERT ON orders
FOR EACH ROW
UPDATE bands
SET stock = stock - 1
WHERE band_id = NEW.band_id;
CREATE TRIGGER tg_ad_orders
AFTER DELETE ON orders
FOR EACH ROW
UPDATE bands
SET stock = stock + 1
WHERE band_id = OLD.band_id;
Her er SQLFiddle demo