framework/db/ActiveQuery.php
protected function queryScalar($selectExpression, $db)
    {
        /* @var $modelClass ActiveRecord */
        $modelClass = $this->modelClass;
        if ($db === null) {
            $db = $modelClass::getDb();
        }

        if ($this->sql === null) {
            return parent::queryScalar($selectExpression, $db);
        }

        $command = (new Query())->select([$selectExpression])
            ->from(['c' => "({$this->sql})"])
            ->params($this->params)
            ->createCommand($db);
        $this->setCommandCache($command);

        return $command->queryScalar();
    }
protected function queryScalar($selectExpression, $db)
    {
        /* @var $modelClass ActiveRecord */
        $modelClass = $this->modelClass;
        if ($db === null) {
            $db = $modelClass::getDb();
        }

        if ($this->sql === null) {
            return parent::queryScalar($selectExpression, $db);
        }

        $command = (new Query())->select([$selectExpression])
            ->from(['c' => "({$this->sql})"])
            ->params($this->params)
            ->createCommand($db);
        $this->setCommandCache($command);

        return $command->queryScalar();
    }
{@inheritdoc}