1.創建表及記錄用于測試

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');

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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)

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2.互換original_price與price的值

新手可能會使用以下方法進行互換

update product set original_price=price,price=original_price;

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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，

然后執行price=original_price，這里相當于沒有更新。

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)

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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;

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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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總結

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