mysql互换表中两列数据方法

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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