I'm designing a database containing messages and messengers. 

```
create table Message(
    MessageID int,
    MessengerID int,
    Content nvarchar(max)
)
create table Messenger(
    MessengerID int,
    MessengerName nvarchar(100)
)

```

Sometimes the messenger in unknown. Would you use a NULL value in that case, or a reserved record for unknown messengers in the Messenger table? I'd love to see a short explanation why one solution is better than the other.