トリガーアプローチを使用する場合は、2つのトリガー(挿入と削除用)が必要です
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;
こちらが