Peter Trast (2/7/2012)
I cannot think of a real world example right now where this would make sense.
Real-world examples are easy enough to find. For instance, the current balance of an account can be calculated from the opening balance (both in the accounts table) and the sum of all tranactions (different table).
But there are technical challenges for allowing this as a computed column. If it's not persisted, any access to the table would automatically involve access to several rows in the other table. And if it is persisted, any change to the table of transactions would involve changing the current balance in the accounts table. This would be a fairly simple change to propagate (which is why you could implement this as an indexed view), but there are lots of scenarios where propagating changes to persisted computed columns becomes a lot more complex. (I know - because I get paid to think about and solve this kind of issues).
Hugo Kornelis, SQL Server MVP
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