Trigger multiply from two tables

问题

i need to create a trigger that multiply two fields from two tables, but i have no idea of how to do it, so let's see if you can help me.

Two tables,

Products (product_name, price)

Orders (product_name (foreign key),units_ordered)

I need to add another field on table the Orders that multiply price from Products and units_ordered from Orders, so : total_price = price (from products) X units_ordered (from Orders)

Thanks in advance, and sorry for my bad english. Regards

回答1:

You don't need trigger at all. And also you don't need to add another column for the total price as it is already redundant.

If you want their total prices, just do it during the projection of records. Example

SELECT  a.Product_Name,
        a.Price,
        b.units_ordered,
        a.Price * b.units_ordered AS TotalPrice 
FROM    Products a
        INNER JOIN Orders b
            ON a.Product_name = b.Product_name

or you can create a VIEW out of your SELECT statement. Example,

CREATE VIEW ProductOrder
AS
SELECT  a.Product_Name,
        a.Price,
        b.units_ordered,
        a.Price * b.units_ordered AS TotalPrice 
FROM    Products a
        INNER JOIN Orders b
            ON a.Product_name = b.Product_name

and selecting from the view,

SELECT * FROM ProductOrder

But if you really want to add another column, still trigger is not an option. You only need to update the values for that column using UPDATE and joining of the two tables. Assuming your new column is called TotalPrice on table `Orders.

UPDATE  Orders a
        INNER JOIN Products b
            ON a.Product_name = b.Product_name
SET     a.TotalPrice = a.units_ordered * b.Price

来源：https://stackoverflow.com/questions/15989477/trigger-multiply-from-two-tables

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mysql

sql

triggers

sql-update