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.

E.g., 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)