yii\db\QueryBuilder::buildUnion PHP Method

    public function buildUnion($unions, &$params)
    {
        if (empty($unions)) {
            return '';
        }
        $result = '';
        foreach ($unions as $i => $union) {
            $query = $union['query'];
            if ($query instanceof Query) {
                list($unions[$i]['query'], $params) = $this->build($query, $params);
            }
            $result .= 'UNION ' . ($union['all'] ? 'ALL ' : '') . '( ' . $unions[$i]['query'] . ' ) ';
        }
        return trim($result);
    }