@Override
	public void decreaseBalance(int id, double amount) {
		double balance = findCustomersBalance(id) - amount;
		String sql = "UPDATE customer SET balance='" + balance 
				+ "' WHERE customer_id='" + id + "';";
		
		Connection con = null;
		try {
			con = getConnection();
			PreparedStatement statement = con.prepareStatement(sql);
			statement.executeUpdate();
		} catch (SQLException e) {
			e.printStackTrace();
		} finally {
			if (con != null) closeConnection(con);
		}
	}