Think\Db\Query::union PHP Method

    public function union($union, $all = false)
    {
        $this->options['union']['type'] = $all ? 'UNION ALL' : 'UNION';
        if (is_array($union)) {
            $this->options['union'] = array_merge($this->options['union'], $union);
        } else {
            $this->options['union'][] = $union;
        }
        return $this;
    }