从另一列计算的列? [英] Column calculated from another column?

问题描述

给定下表:

id | value
--------------
1     6
2     70

有没有办法添加一个基于同一个表中的另一列自动计算的列?类似于 VIEW，但属于同一个表的一部分.例如，calculated 将是 value 的一半.Calculated 应该在 value 更改时自动更新，就像 VIEW 一样.

Is there a way to add a column that is automatically calculated based on another column in the same table? Like a VIEW, but part of the same table. As an example, calculated would be half of value. Calculated should be automatically updated when value changes, just like a VIEW would be.

结果是:

id | value | calculated
-----------------------
1     6       3
2     70      35

推荐答案

Generated Column 是 MySql 5.7.6 及以上版本的好方法之一.

Generated Column is one of the good approach for MySql version which is 5.7.6 and above.

生成的列有两种:

• 虚拟(默认) - 列将在运行时计算从表中读取记录
• Stored - 列将在在表中写入/更新新记录

两种类型都可以有 NOT NULL 限制，但只有存储的 Generated Column 可以是索引的一部分.

Both types can have NOT NULL restrictions, but only a stored Generated Column can be a part of an index.

对于当前情况，我们将使用存储的生成列.为了实现，我认为计算所需的两个值都存在于表中

For current case, we are going to use stored generated column. To implement I have considered that both of the values required for calculation are present in table

CREATE TABLE order_details (price DOUBLE, quantity INT, amount DOUBLE AS (price * quantity));

INSERT INTO order_details (price, quantity) VALUES(100,1),(300,4),(60,8);

amount 会自动弹出到表格中，您可以直接访问它，另外请注意，每当您更新任何列时，amount 也会更新.

amount will automatically pop up in table and you can access it directly, also please note that whenever you will update any of the columns, amount will also get updated.

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