SQL Server computed column select from another tab

问题:

I'm not to sure what the best way to go about this is, so i'll describe what the end goal is, and if a computed column is the answer then please help me go that route, or perhaps a better route.

I have two tables:

Orders

OrderId
PackageId
MediaSpend
TotalAdViews (Computed column)

Packages

PackageId
BaseAdViews

Each order is assigned a package that comes with say 1,000 views, you can then buy more media spend to get more views. I wanted to create a column called TotalAdViews which would add BaseAdViews + MediaSpend. From my understanding if persistance is enabled the column won't need to recalculate every time it is queried, which could help performance.

How do I get a value from another table in my computed column? Or please suggest an alternate way of accomplishing my goal.

回答1:

You won't be able to use columns from another table within a computed column expression. This is an extract from the MSDN documentation.

A computed column is computed from an expression that can use other columns in the same table.

You mentioned that your motivation for using a computed column was to increase performance. There are a lot of restrictions but an indexed view might add value here.

回答2:

I know this answer comes two years late, but just to help anyone who googles and finds this post:

It is perfectly legal to define a user-defined function and use it as the computed value. This function may contain select statements from other tables.

CREATE FUNCTION dbo.getSumAdViews(@packageId int)
RETURNS TABLE
AS
RETURN
    select SUM(BaseAdViews) from Packages where PackageId = @packageId

Then in your computed column, just use the expression dbo.getSumAdViews(PackageId)

回答3:

if it is only for display purposes,why not create a view..

select <<add other columns you need>> mediaspend+basicadviews as totaladviews
from 
orders o
join
packages p
on p.packageid=o.orderid