1.创建表及记录用于测试
CREATE TABLE `product` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '产品id', `name` varchar(50) NOT NULL COMMENT '产品名称', `original_price` decimal(5,2) unsigned NOT NULL COMMENT '原价', `price` decimal(5,2) unsigned NOT NULL COMMENT '现价', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; INSERT INTO `product` (`id`, `name`, `original_price`, `price`) VALUES (NULL, '雪糕', '5', '3.5'), (NULL, '鲜花', '18', '15'), (NULL, '甜点', '25', '12.5'), (NULL, '玩具', '55', '45'), (NULL, '钱包', '285', '195');
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mysql> select * from product; +----+--------+----------------+--------+ | id | name | original_price | price | +----+--------+----------------+--------+ | 1 | 雪糕 | 5.00 | 3.50 | | 2 | 鲜花 | 18.00 | 15.00 | | 3 | 甜点 | 25.00 | 12.50 | | 4 | 玩具 | 55.00 | 45.00 | | 5 | 钱包 | 285.00 | 195.00 | +----+--------+----------------+--------+ 5 rows in set (0.00 sec)
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2.互换original_price与price的值
新手可能会使用以下方法进行互换
update product set original_price=price,price=original_price;
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但这样执行的结果只会使original_price与price的值都是price的值，因为update有顺序的，
先执行original_price=price , original_price的值已经更新为price，
然后执行price=original_price，这里相当于没有更新。
执行结果：
mysql> select * from product; +----+--------+----------------+--------+ | id | name | original_price | price | +----+--------+----------------+--------+ | 1 | 雪糕 | 5.00 | 3.50 | | 2 | 鲜花 | 18.00 | 15.00 | | 3 | 甜点 | 25.00 | 12.50 | | 4 | 玩具 | 55.00 | 45.00 | | 5 | 钱包 | 285.00 | 195.00 | +----+--------+----------------+--------+ 5 rows in set (0.00 sec) mysql> update product set original_price=price,price=original_price; Query OK, 5 rows affected (0.00 sec) Rows matched: 5 Changed: 5 Warnings: 0 mysql> select * from product; +----+--------+----------------+--------+ | id | name | original_price | price | +----+--------+----------------+--------+ | 1 | 雪糕 | 3.50 | 3.50 | | 2 | 鲜花 | 15.00 | 15.00 | | 3 | 甜点 | 12.50 | 12.50 | | 4 | 玩具 | 45.00 | 45.00 | | 5 | 钱包 | 195.00 | 195.00 | +----+--------+----------------+--------+ 5 rows in set (0.00 sec)
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正确的互换方法如下：
update product as a, product as b set a.original_price=b.price, a.price=b.original_price where a.id=b.id;
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执行结果：
mysql> select * from product; +----+--------+----------------+--------+ | id | name | original_price | price | +----+--------+----------------+--------+ | 1 | 雪糕 | 5.00 | 3.50 | | 2 | 鲜花 | 18.00 | 15.00 | | 3 | 甜点 | 25.00 | 12.50 | | 4 | 玩具 | 55.00 | 45.00 | | 5 | 钱包 | 285.00 | 195.00 | +----+--------+----------------+--------+ 5 rows in set (0.00 sec) mysql> update product as a, product as b set a.original_price=b.price, a.price=b.original_price where a.id=b.id; Query OK, 5 rows affected (0.01 sec) Rows matched: 5 Changed: 5 Warnings: 0 mysql> select * from product; +----+--------+----------------+--------+ | id | name | original_price | price | +----+--------+----------------+--------+ | 1 | 雪糕 | 3.50 | 5.00 | | 2 | 鲜花 | 15.00 | 18.00 | | 3 | 甜点 | 12.50 | 25.00 | | 4 | 玩具 | 45.00 | 55.00 | | 5 | 钱包 | 195.00 | 285.00 | +----+--------+----------------+--------+ 5 rows in set (0.00 sec)
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