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;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
;
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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