关于mysql互换表中两列数据方法的讲解

1.创建表及记录用于测试

CREATE TABLE `product` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT &＃39;产品id&＃39;, `name` varchar(50) NOT NULL COMMENT &＃39;产品名称&＃39;, `original_price` decimal(5,2) unsigned NOT NULL COMMENT &＃39;原价&＃39;, `price` decimal(5,2) unsigned NOT NULL COMMENT &＃39;现价&＃39;, PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `product` (`id`, `name`, `original_price`, `price`) VALUES (NULL, &＃39;雪糕&＃39;, &＃39;5&＃39;, &＃39;3.5&＃39;), 
(NULL, &＃39;鲜花&＃39;, &＃39;18&＃39;, &＃39;15&＃39;), 
(NULL, &＃39;甜点&＃39;, &＃39;25&＃39;, &＃39;12.5&＃39;), 
(NULL, &＃39;玩具&＃39;, &＃39;55&＃39;, &＃39;45&＃39;), 
(NULL, &＃39;钱包&＃39;, &＃39;285&＃39;, &＃39;195&＃39;);

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: 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)

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: 0mysql> 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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