互換資料表中兩列資料方法

2021-07-13 03:32:11 字數 4105 閱讀 3805

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)

新手可能會使用以下方法進行互換

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,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;

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)

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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...