MySQL错误代码： 1062Duplicate entry ‘0‘ for key ‘PRIMARY‘

USE purchase;
#(1) 构建product表的主键为product_id, sort表的主键为sort_id, subsort表的主键为subsort_id
ALTER TABLE product
MODIFY product_id INT(5) PRIMARY KEY;
DESC product;
#报错说有重复的0
#发现有很多空值
SELECT `Product_ID`
FROM `product`
WHERE `Product_ID` IS NULL;
#删除空值
DELETE
FROM `product`
WHERE `Product_ID` IS NULL;
#又报错说`Product_ID`有重复的33
SELECT * FROM `product`
WHERE product_id =33;
#我把两个33都删了，就有自己给添了一行33
INSERT INTO `product`(`Product_ID`,`Product_Name`,`Product_Code`,`Product_Date`,`Price`,`Unit`,`Detail`,`SubSort_ID`,`Sort_ID`)
VALUES(33,'理光3813复印机','1101033','2016-10-20',9800.00,'台','1*1','1101','11');
#又报错说`Product_ID`有重复的44
SELECT * FROM `product`
WHERE product_id =44;

ALTER TABLE product
MODIFY product_id INT(5) PRIMARY KEY;
DESC product;