mysql互換表中兩列數據方法

1.建立表及記錄用於測試

CREATE TABLE product (
id int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '產品id',
name varchar(50) NOT NULL COMMENT '產品名稱',
original_price decimal(5,2) unsigned NOT NULL COMMENT '原價',
price decimal(5,2) 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

新手可能會使用如下方法進行互換

update product set original_price=price,price=original_price;

但這樣執行的結果只會使original_price與price的值都是price的值，由於update有順序的，
先執行original_price=price , original_price的值已經更新爲price，
而後執行price=original_price，這裏至關於沒有更新。

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)

正確的互換方法以下：

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

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)