[英]Changing the value of a table field after it has already been inserted
I am working with a CMS system, and using a kind of smartform am inserting fields into a table. 我正在使用CMS系统,并使用一种smartform将字段插入表中。
In these fields are the name of a product, quantity and cost. 在这些字段中,是产品名称,数量和成本。
cost and product name are taken from a products table, while quantity is input manually. 成本和产品名称从产品表中获取,而数量则手动输入。
I understand this is not the best architecture, but I am limited with the system I am working with. 我知道这不是最好的体系结构,但是我所使用的系统受到限制。
What I want to do, is change the value of cost after it is inserted, to quantity * cost. 我要做的是将插入后的成本值更改为数量*成本。
What is the easiest way to do this? 最简单的方法是什么?
To change one row: 更改一行:
update TheTable
set cost = quantity * cost
where pk = 123
If you run this multiple times, costs will explode ;-) 如果您多次运行,成本将激增;-)
You can run an UPDATE
after the insert, using the identity field of the new product: 您可以在插入后使用新产品的标识字段运行
UPDATE
:
UPDATE products
SET cost = cost * quantity
WHERE product_id = @productId
If you have your hands tied so tightly that you are unable to change the table structure, then you probably won't be able to use a trigger, but here goes anyway. 如果您的双手绑得太紧,以至于无法更改表的结构,那么您可能将无法使用触发器,但是无论如何都可以使用触发器。
This is a sample table structure: 这是一个示例表结构:
DROP TABLE IF EXISTS `products`;
CREATE TABLE `products` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`product` VARCHAR(255),
`quantity` INT(11) NOT NULL,
`cost` DECIMAL(6,2) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
Create a trigger which updates the cost prior to it being inserted into the products
table: 创建一个触发器,该触发器在将成本插入
products
表之前对其进行更新:
DELIMITER //
DROP TRIGGER IF EXISTS update_cost//
CREATE TRIGGER update_cost BEFORE INSERT ON products
FOR EACH ROW BEGIN
SET NEW.cost = NEW.cost * NEW.quantity;
END;
//
DELIMITER ;
Insert a couple of products into the table: 将几个产品插入表中:
INSERT INTO `products` (`product`, `quantity`, `cost`) VALUES
('Acme portable hole', 10, 20),
('Acme jet pack', 1000, 2);
The cost is updated automatically: 费用会自动更新:
SELECT * FROM prodcuts;
+----+--------------------+----------+---------+
| id | product | quantity | cost |
+----+--------------------+----------+---------+
| 1 | Acme portable hole | 10 | 200.00 |
| 2 | Acme jet pack | 1000 | 2000.00 |
+----+--------------------+----------+---------+
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