Sorry for the long question title.
I guess I'm on to a loser on this one but on the off chance.
Is it possible to make the calculation of a calculated field in a table the result of an aggregate function applied to a field in another table.
i.e.
You have a table called 'mug', this has a child called 'color' (which makes my UK head hurt but the vendor is from the US, what you going to do?) and this, in turn, has a child called 'size'. Each table has a field called sold.
The size.sold increments by 1 for every mug of a particular colour and size sold.
You want color.sold to be an aggregate of SUM size.sold WHERE size.colorid = color.colorid
You want mug.sold to be an aggregate of SUM color.sold WHERE color.mugid = mug.mugid
Is there anyway to make mug.sold and color.sold just work themselves out or am I going to have to go mucking about with triggers?
you can't have a computed column directly reference a different table, but you can have it reference a user defined function. here's a link to a example of implementing a solution like this.
http://www.sqlservercentral.com/articles/User-Defined+functions/complexcomputedcolumns/2397/
No, it is not possible to do this. A computed column can only be derived from the values of other fields on the same row. To calculate an aggregate off another table you need to create a view.
If your application needs to show the statistics ask the following questions:
- Is it really necessary to show this in real time? If so, why? If it is really necesary to do this, then you would have to use triggers to update a table. This links to a short wikipedia article on denormalisation. Triggers will affect write performance on table updates and relies on the triggers being active.
- If it is only necessary for reporting purposes, you could do the calculation in a view or a report.
- If it is necessary to support frequent ad-hoc reports you may be into the realms of a data mart and overnight ETL process.
来源：https://stackoverflow.com/questions/166518/sql-server-2005-computed-column-result-from-aggregate-of-another-table-fields-v