// SELECT * FROM (SELECT * FROM `user` WHERE `active` = 1) `activeusers`;
$subquery = (new \yii\db\Query)->from('user')->where(['active' => true])
$query = (new \yii\db\Query)->from(['activeusers' => $subquery]);
// subquery can also be a string with plain SQL wrapped in parenthesis
// SELECT * FROM (SELECT * FROM `user` WHERE `active` = 1) `activeusers`;
$subquery = "(SELECT * FROM `user` WHERE `active` = 1)";
$query = (new \yii\db\Query)->from(['activeusers' => $subquery]);