framework/db/Query.php
public function scalar($db = null)
    {
        if ($this->emulateExecution) {
            return null;
        }

        return $this->createCommand($db)->queryScalar();
    }
public function scalar($db = null)
    {
        if ($this->emulateExecution) {
            return null;
        }

        return $this->createCommand($db)->queryScalar();
    }
Returns the query result as a scalar value.
The value returned will be the first column in the first row of the query results.
@param Connection $db the database connection used to generate the SQL statement.
If this parameter is not given, the `db` application component will be used.
@return string|null|false the value of the first column in the first row of the query result.
False is returned if the query result is empty.