This QotD raises a point I'm always having to remind developers about, and I guess SQL guys can also do with a reminder from time to time.

However, my 2 cents is rather than...

declare @s-2 char(4)

select ISNULL(convert(varchar(16),@s),'Unknown')

...I would use...

declare @s-2 char(4)

select coalesce(@s,'Unknown')

...which also has the ability to take more than 2 arguments.

S.