如何根据公式填充mysql列值? [英] how to populate mysql column value based on a formula?

问题描述

我用以下几列创建了一个mysql表...

I have created a mysql table with the following columns...

item_price, discount, delivery_charge, grand_total

item_price，discount和delivery_charge的值都根据所考虑的项目而有所不同，这些将手动填写到表中.

The value of item_price, discount, and delivery_charge all vary depending on the item under consideration, and these would be filled into the table manually.

我想让mysql使用以下公式根据item_price，discount和delivery_charge的值填充grand_total列中的值.

I would like to have mysql populate the value in the grand_total column based on the values of item_price, discount, and delivery_charge, using the formula below...

grand_total = item_price - discount + delivery_charge

我可以在mysql中创建表结构时以某种方式为此列指定公式吗?我知道可以使用php完成此操作，但我更喜欢数据库(如果可能)自动为我完成操作.

Can I somehow specify a formula for this column while creating the table structure in mysql? I know this can be done using php, but I prefer the database doing this for me automatically, if possible.

推荐答案

以下是将起作用的触发器的示例.请注意，由于临时更新"的要求，您既需要更新触发器，又需要插入触发器.

Here's an example of a trigger that will work. Note that you'll need both update and insert triggers due to your "ad hoc update" requirements.

delimiter ~

create trigger my_table_update before update on my_table
for each row begin
    set new.grand_total = new.item_price - new.discount + new.delivery_charge;
end~

create trigger my_table_insert before insert on my_table
for each row begin
    set new.grand_total = new.item_price - new.discount + new.delivery_charge;
end~

delimiter ;

但是视图会不会变得更简单更好?

But wouldn't a view be simpler and better?

create view my_table_view as
select item_price, discount, delivery_charge, 
item_price - discount + delivery_charge as grand_total
from my_table;

然后仅从视图中选择而不是从表中选择

then just select from the view instead of the table

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