protected function where_clause_list($field, array $values) {
        if (empty($values)) {
            return array("1 = 2", array()); // Fake condition, won't return rows ever. MDL-17645
        }

        // Note: Do not use get_in_or_equal() because it can not deal with bools and nulls.

        $params = array();
        $select = "";
        $values = (array)$values;
        foreach ($values as $value) {
            if (is_bool($value)) {
                $value = (int)$value;
            }
            if (is_null($value)) {
                $select = "$field IS NULL";
            } else {
                $params[] = $value;
            }
        }
        if ($params) {
            if ($select !== "") {
                $select = "$select OR ";
            }
            $count = count($params);
            if ($count == 1) {
                $select = $select."$field = ?";
            } else {
                $qs = str_repeat(',?', $count);
                $qs = ltrim($qs, ',');
                $select = $select."$field IN ($qs)";
            }
        }
        return array($select, $params);
    }