INSERTAR y ACTUALIZAR en una consulta MYSQL

Si opta por un enfoque de activación, necesitará dos activadores (para insertar y eliminar)

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;

Aquí está SQLFiddle demostración