protected function handleJoinConditions(array $conditions)
    {
        if (empty($conditions)) {
            return '';
        }
        $sql[] = $this->{$conditions[0]['type']}($conditions[0]);
        $count = count($conditions);
        for ($i = 1; $i < $count; $i++) {
            $sql[] = $conditions[$i]['separator'] . ' ' . $this->{$conditions[$i]['type']}($conditions[$i]);
        }
        return implode(' ', $sql);
    }