Think\Db\Builder::parseUnion PHP Метод

    protected function parseUnion($union)
    {
        if (empty($union)) {
            return '';
        }
        $type = $union['type'];
        unset($union['type']);
        foreach ($union as $u) {
            if ($u instanceof \Closure) {
                $sql[] = $type . ' ' . $this->parseClosure($u, false);
            } elseif (is_string($u)) {
                $sql[] = $type . ' ' . $this->parseSqlTable($u);
            }
        }
        return implode(' ', $sql);
    }