如何将计算列添加到需要连接语句的表中？

Question

I cant figure out my syntax issue. I am trying to ADD a computed column to an orders table. In order to do that I need to join 2 tables (Orders and Product).

ALTER TABLE tblOrder_Product
ADD [TotalCost] money 
AS (SELECT (([tbop].[Quantity])*([P].[ProductPrice]) as TotalCost)
FROM tblOrder_Product tbop INNER JOIN Product P ON tbop.ProductID = P.ProductID
GROUP BY TotalCost)

Answer 1

You can only add a computed column based on values in that table. You can create your own logical computed column if you want to create triggers to manage the value and in the trigger code you can reference the other table, but triggers are not generally recommended.

Answer 2

If you can write a scalar valued function that takes only values from your table and returns the value you're looking for, you can define the computed column with that. For instance, in your case:

create function dbo.fn_TotalCost(@quantity int)
returns float
as
begin

    declare @r float;
    SELECT @r = @quantity*[P].[ProductPrice]
    FROM tblOrder_Product tbop INNER JOIN Product P ON tbop.ProductID = P.ProductID
    GROUP BY TotalCost;

    return @r;
end
go
alter table dbo.yourTable 
   add TotalCost as dbo.fnTotalCost(ProductID, Quantity);