At least MSSQL uses the UPDATE/UPDATE_COLUMNS functions' approach.

[]s

>A more general solution would be a comparison operator that returned
>true if the two operands were equivalent -- either equal in value or
>both missing. This would avoid a special case in a trigger while making
>a useful construct available in all contexts. I'd be surprised if some
>other database systems didn't already have such an operator; if so, we
>should consider their choice of operator. If I were a less cynical
>person, I'd suggest "==".