MySQL function to convert ISBN10 to ISBN13

ISBN10to13.sql

DROP FUNCTION IF EXISTS `ISBN10to13`;
delimiter //
CREATE FUNCTION `ISBN10to13`(isbn10 VARCHAR(50)) RETURNS varchar(50) CHARSET utf8
BEGIN
    DECLARE isbn13 VARCHAR(13);
    DECLARE i   INT;
    DECLARE chk INT;

    IF (LENGTH(ISBN10) > 10) THEN 
        RETURN ISBN10;
    ELSE 
        SET isbn10=SUBSTRING(ISBN10,1,10);
    END IF;

    # set ISBN10    = '0123456479';
    SET isbn13  = CONCAT('978' , LEFT(isbn10, 9));
    SET i = 1, chk  = 0;

    # 9*1+7*3+8*1=38
    SET chk = (38 + 3*LEFT(isbn10,1) 
        + RIGHT(LEFT(isbn10,2),1)
        + 3*RIGHT(LEFT(isbn10,3),1)
        + RIGHT(LEFT(isbn10,4),1)
        + 3*RIGHT(LEFT(isbn10,5),1)
        + RIGHT(LEFT(isbn10,6),1)
        + 3*RIGHT(LEFT(isbn10,7),1) 
        + RIGHT(LEFT(isbn10,8),1) 
        + 3*LEFT(RIGHT(isbn10,2),1));

    SET chk = 10 - (chk % 10);
    IF (chk<>10) then
        SET isbn13 = concat(isbn13 , CONVERT(chk, CHAR(1)));
    ELSE
        SET isbn13 = concat(isbn13 , '0');
    END IF;
    RETURN isbn13;
END //
delimiter ;