MYSQL 多重判断 NULL 的问题

BEGIN
DECLARE done INT DEFAULT 0;
DECLARE scoreNumber INT DEFAULT 0;
DECLARE DefstudentId INT DEFAULT 0;
DECLARE backupScrore double DEFAULT 0;
DECLARE cur1 CURSOR FOR select sum(scoreNum) as sumNum,passivityId as studentId from Score group by passivityId ;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

UPDATE student d set d.badge = null;

OPEN cur1;

REPEAT
FETCH cur1 into scoreNumber,DefstudentId;
IF done = 0 THEN

SELECT  score INTO backupScrore from `backup`  WHERE studentId = DefstudentId ORDER BY backupdate desc ,backupId DESC LIMIT 1;   

IF backupScrore is not NULL THEN
UPDATE student d set d.growScore= scoreNumber- backupScrore where  d.studentId= DefstudentId;                                           
END IF;

END IF;

UNTIL done = 1 
END REPEAT;

CLOSE cur1;

END

本来是想判断 游标 值为空的时候跳出循环
DECLARE cur1 CURSOR FOR select sum(scoreNum) as sumNum,passivityId as studentId from Score group by passivityId ;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
结果在中间 因为 这一句
IF backupScrore is not NULL THEN
UPDATE student d set d.growScore= scoreNumber- backupScrore where d.studentId= DefstudentId;

END IF;
就因为 backupScrore 为空就跳出了循环 该怎么改才能让这个 存储过程正确运行?

参考我这个存贮过程:[url]http://zyn010101.iteye.com/blog/1489539[/url]