Update with math using new values?

update_math_test.php

<?php
/**
*
* @package testing
* @copyright (c) 2012 phpBB Group
* @license http://opensource.org/licenses/gpl-2.0.php GNU General Public License v2
*
*/

require_once dirname(__FILE__) . '/../../phpBB/includes/functions.php';

class phpbb_dbal_update_math_test extends phpbb_database_test_case
{
	public function getDataSet()
	{
		return $this->createXMLDataSet(dirname(__FILE__).'/fixtures/update_math_test.xml');
	}

	public function test_update_math()
	{
		$db = $this->new_dbal();

		$sql = 'UPDATE phpbb_users
			SET user_lastvisit = 8,
				user_lastmark = 16,
				user_lastpost_time = user_lastvisit + user_lastmark';
		$db->sql_query($sql);

		$sql = 'SELECT user_lastpost_time
			FROM phpbb_users';
		$result = $db->sql_query($sql);
		$this->assertEquals(24, $db->sql_fetchfield('user_lastpost_time'));
		$db->sql_freeresult($result);
	}
}

update_math_test.xml

<?xml version="1.0" encoding="UTF-8" ?>
<dataset>
	<table name="phpbb_users">
		<column>user_id</column>
		<column>user_lastvisit</column>
		<column>user_lastmark</column>
		<column>user_lastpost_time</column>
		<column>user_permissions</column>
		<column>user_sig</column>
		<column>user_occ</column>
		<column>user_interests</column>
		<row>
			<value>1</value>
			<value>1</value>
			<value>2</value>
			<value>4</value>
			<value></value>
			<value></value>
			<value></value>
			<value></value>
		</row>
	</table>
</dataset>