mysql互換表中兩列資料方法

1.建立表及記錄用于測試

CREATE TABLE `product` (
 `id` int() unsigned NOT NULL AUTO_INCREMENT COMMENT '産品id',
 `name` varchar() NOT NULL COMMENT '産品名稱',
 `original_price` decimal(,) unsigned NOT NULL COMMENT '原價',
 `price` decimal(,) unsigned NOT NULL COMMENT '現價',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `product` (`id`, `name`, `original_price`, `price`) VALUES 
(NULL, '雪糕', '5', '3.5'), 
(NULL, '鮮花', '18', '15'), 
(NULL, '甜點', '25', '12.5'), 
(NULL, '玩具', '55', '45'), 
(NULL, '錢包', '285', '195');
           

mysql> select * from product;
+----+--------+----------------+--------+
| id | name   | original_price | price  |
+----+--------+----------------+--------+
|  1 | 雪糕   |           5.00 |   3.50 |
|  2 | 鮮花   |          18.00 |  15.00 |
|  3 | 甜點   |          25.00 |  12.50 |
|  4 | 玩具   |          55.00 |  45.00 |
|  5 | 錢包   |         285.00 | 195.00 |
+----+--------+----------------+--------+
5 rows in set (0.00 sec)
           

2.互換original_price與price的值

新手可能會使用以下方法進行互換

但這樣執行的結果隻會使original_price與price的值都是price的值，因為update有順序的，

先執行original_price=price , original_price的值已經更新為price，

然後執行price=original_price，這裡相當于沒有更新。

執行結果：

mysql> select * from product;
+----+--------+----------------+--------+
| id | name   | original_price | price  |
+----+--------+----------------+--------+
|  1 | 雪糕   |           5.00 |   3.50 |
|  2 | 鮮花   |          18.00 |  15.00 |
|  3 | 甜點   |          25.00 |  12.50 |
|  4 | 玩具   |          55.00 |  45.00 |
|  5 | 錢包   |         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 | 鮮花   |          15.00 |  15.00 |
|  3 | 甜點   |          12.50 |  12.50 |
|  4 | 玩具   |          45.00 |  45.00 |
|  5 | 錢包   |         195.00 | 195.00 |
+----+--------+----------------+--------+
5 rows in set (0.00 sec)
           

正确的互換方法如下：

執行結果：

mysql> select * from product;
+----+--------+----------------+--------+
| id | name   | original_price | price  |
+----+--------+----------------+--------+
|  1 | 雪糕   |           5.00 |   3.50 |
|  2 | 鮮花   |          18.00 |  15.00 |
|  3 | 甜點   |          25.00 |  12.50 |
|  4 | 玩具   |          55.00 |  45.00 |
|  5 | 錢包   |         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 | 鮮花   |          15.00 |  18.00 |
|  3 | 甜點   |          12.50 |  25.00 |
|  4 | 玩具   |          45.00 |  55.00 |
|  5 | 錢包   |         195.00 | 285.00 |
+----+--------+----------------+--------+
5 rows in set (0.00 sec)