SQL用户存在则更新不存在则删除

alter table T_Cart add unique index(goods_id,user_id);

/*
	 * 保存购物车(如果不存在则插入,如果存在则更新)
	 */
	@Override
	public void insetCart(Cart cart) throws Exception {
		String sql = "INSERT INTO T_Cart(goods_id,order_num,user_id) VALUES(?,?,?) ON DUPLICATE KEY UPDATE order_num=order_num+1";
		util.update(sql, cart.getGoods_id(), 1, cart.getUser_id());
	}