Eine Erklärung der zweispaltigen Datenmethode in der MySQL-Austauschtabelle

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)

update product set original_price=price,price=original_price;

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: 0mysql> 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)

update product as a, product as b set a.original_price=b.price, a.price=b.original_price where a.id=b.id;