MySQL function to convert ISBN13 to ISBN10

ISBN13to10.sql

DROP FUNCTION IF EXISTS `ISBN13to10`;
delimiter //
CREATE FUNCTION `ISBN13to10`(isbn13 VARCHAR(50)) RETURNS varchar(50) CHARSET utf8
BEGIN
    DECLARE isbn10 	VARCHAR(13);
    DECLARE i   	VARCHAR(13);
    DECLARE sum 	INT;
    DECLARE chk 	INT;
    DECLARE chkchar 	VARCHAR(3);

    IF (LENGTH(isbn13) = 10) THEN
        RETURN isbn13;
    ELSEIF (LENGTH(isbn13) != 13 || SUBSTRING(isbn13, 1, 3) != '978') THEN
        RETURN isbn13;
    END IF;

    SET i = SUBSTRING(isbn13, 4, 10);

    IF (LENGTH(i) < 10) THEN
	SET i = LPAD(i, 10 , '0');
    END IF;
	
    SET sum = 
        1 * LEFT(i ,1) 
        + 2 * RIGHT(LEFT(i ,2),1)
        + 3 * RIGHT(LEFT(i ,3),1)
        + 4 * RIGHT(LEFT(i ,4),1)
        + 5 * RIGHT(LEFT(i ,5),1)
        + 6 * RIGHT(LEFT(i ,6),1)
        + 7 * RIGHT(LEFT(i ,7),1) 
        + 8 * RIGHT(LEFT(i ,8),1) 
        + 9 * RIGHT(LEFT(i ,9),1);   

	SET chkchar = 'X';	
    SET chk = sum % 11;
    
    IF (chk = 10) THEN
        SET chk = 90980;        
    END IF;
	
	IF (chk != 90980) THEN
		SET chkchar = CAST(chk as CHAR);
	END IF;
	
    RETURN CONCAT(SUBSTRING(isbn13, 4, 9), chkchar);
END //
delimiter ;