Having trouble setting a computed column as not Null

I'm having problems setting a computed column as not null .

What I want to achieve is C001 , C002... , etc. and at the same time set it as not null .

I have read on a forum that this can be achieved by using the default value 0 for NULL values.
Eg, ISNULL(Price + Taxes, 0)

I have tried to apply to this formula:

('C'+right('000'+CONVERT([varchar](3),[ID],(0)),(3)))

But it didn't seem to work. Can anyone tell me what am I missing?

ALTER CreditCard accountNo AS ISNULL('C'+right('000'+CONVERT([varchar](3),[idCreditCard],(0)),(3)),0)

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I've finally found the solution to my problem!

The correct query should be:

ALTER TABLE CreditCard ADD accountNo AS ISNULL('C'+right('000'+CONVERT([varchar](3),[idCreditCard],(0)),(3)),0)

Thanks for the help guys!

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If this relates to SQL Server, you might be interested in this from MSDN.

'NOT NULL can be specified for computed columns only if PERSISTED is also specified.' http://msdn.microsoft.com/en-us/library/ms190273.aspx

... but after trying to reverse-engineering the question from the answer, I think that 'keenlearner wanted only to ensure that there was never a null value in the column without having the constraint.

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