Create Computed Column using data from another table

Question

I have a SQL Server 2008 R2 database. This database has two tables called Pictures and PictureUse.

Picture table has the following columns:

Id (int)          


        

Answer 1

A computed column may only reference other columns in the same table. You could (as per jeroenh's answer) use a UDF, but the column won't be stored or be indexable and so it has to be recomputed every time the row is accessed.

You could create an indexed view that contains this information (if, as I suspect, it's just the count of rows from PictureUse):

CREATE VIEW dbo.PictureStats
WITH SCHEMABINDING
AS
    SELECT PictureID,COUNT_BIG(*) as Cnt from dbo.PictureUse
GO
CREATE UNIQUE CLUSTERED INDEX IC_PictureStats on dbo.PictureStats (PictureID)

Behind the scenes, SQL Server will effectively create a table that contains the results of this view, and every insert, update or delete to PictureUse will maintain this results table automatically for you.