yii\db\ActiveQuery::queryScalar PHP Method

    protected function queryScalar($selectExpression, $db)
    {
        if ($this->sql === null) {
            return parent::queryScalar($selectExpression, $db);
        }
        /* @var $modelClass ActiveRecord */
        $modelClass = $this->modelClass;
        if ($db === null) {
            $db = $modelClass::getDb();
        }
        return (new Query())->select([$selectExpression])->from(['c' => "({$this->sql})"])->params($this->params)->createCommand($db)->queryScalar();
    }

 protected function queryScalar($selectExpression, $db)
 {
     $this->_skipPrep = true;
     $r = parent::queryScalar($selectExpression, $db);
     $this->_skipPrep = false;
     return $r;
 }