Union

The Yiisoft\Db\Query\Query::union() method specifies the UNION fragment of a SQL query.

For example:

use Yiisoft\Db\Connection\ConnectionInterface;
use Yiisoft\Db\Query\Query;

/** @var ConnectionInterface $db */

$query1 = $db->select("id, category_id AS type, name")->from('{{%post}}')->limit(10);
$query2 = $db->select('id, type, name')->from('{{%user}}')->limit(10);
$query1->union($query2);

Further calls to Yiisoft\Db\Query\Query::union() will append other UNION fragments.