SELECT * FROM `client` WHERE `subnet` IN (SELECT `id` FROM `subnet` WHERE `bras` = 2)
$subnetQuery = Subnet::find();
$subnetQuery->andWhere(['bras' => 2]);
$clientQuery = Clint::find();
$clientQuery->andWehere(['in', 'subnet', $subnetQuery]);
//
$clientQuery->all();
//
$clientQuery->limit(1);
$clientQuery->one();