Minha solução

DROP PROCEDURE IF EXISTS get_invoice;

DELIMITER $$

CREATE PROCEDURE get_invoice (IN pMonth INT, IN pYear INT)
BEGIN
    DECLARE recordCount INT;
    DECLARE isEndOfCursor BOOLEAN DEFAULT FALSE;
    DECLARE quantitySold INT DEFAULT 0;
    DECLARE itemPrice FLOAT DEFAULT 0;
    DECLARE totalRevenue FLOAT DEFAULT 0;
    DECLARE cur CURSOR FOR 
        SELECT A.quantity, A.price 
        FROM items_sold A 
        INNER JOIN sales_notes B
        ON A.sale_number = B.number
        WHERE MONTH(B.sale_date) = pMonth AND YEAR(B.sale_date) = pYear;
        
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET isEndOfCursor = TRUE;
    
    SET recordCount = (SELECT COUNT(*) FROM sales_notes 
                        WHERE MONTH(sale_date) = pMonth AND YEAR(sale_date) = pYear);
                        
    IF recordCount > 0 THEN
        OPEN cur;
        
        WHILE (isEndOfCursor = FALSE) DO
            FETCH cur INTO quantitySold, itemPrice;
            IF (isEndOfCursor IS FALSE) THEN 
                SET totalRevenue = totalRevenue + (quantitySold * itemPrice);
            END IF;
        END WHILE;
        
        CLOSE cur;
        
        SELECT CONCAT('Total Revenue for ', pMonth, ' ', pYear, ': $', totalRevenue) AS Total_Revenue;
    ELSE
        SELECT 'No records found for the specified month and year.' AS Message;
    END IF;
END $$

DELIMITER ;

CALL get_invoice(09, 2023);