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");

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)

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;

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)

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;

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)