Not in one...

Now, you didn't specify what database product that you're using. This will work for SQL Server 2000.

DECLARE @newOrderKey TABLE (
 newKey int IDENTITY(1,1),
 oldKey int 
)

INSERT INTO @newOrderKey (oldKey)
SELECT OrderKey FROM sometable ORDER BY OrderKey ASC

UPDATE sometable
SET OrderKey = (SELECT newKey FROM @newOrderKey nk WHERE nk.oldKey = sometable.OrderKey)

OK, you can do it in one query but the performance will be horrible. It requires counting the distinct OldKey values prior the the current OldKey value:

UPDATE sometable
SET OrderKey = (SELECT COUNT(DISTINCT OrderKey)+1 FROM sometable st2 WHERE st2.OrderKey < sometable.OrderKey)

Be wary of this one. It's really going to depend on how sophisticated the query processor is. SQL Server should be able to handle it.

-MIke