>>update States set State = 'SP' where Len(State) = 0 && Empty state
>
>I am not sure that is such a good idea.
>
>len(null) returns .null.
>len(' ') returns 2
>empty(null) returns .f.
>
>In this case I use empty(nvl(state,""))

You're right, I was thinking about SQL Server. Although in SQL Server LEN(NULL) will return null.