On the topic of few things being impossible in ServiceNow, let's discuss one-to-one relationships.
Strictly speaking, a one-to-one relationship doesn't truly exist in ServiceNow. In database parlance, this would require that the right-hand table records have a primary key which matches the primary key of a record in the left table. Thus, you could have a left-hand record without a right-hand one, but could never have a right-hand record without the left-hand one.
That's interesting, but ServiceNow's Sys IDs are unique, and they have to be, because of the way ServiceNow's databases are structured on the back-end. Technically, ServiceNow has a flat database structure, depending on how it's configured. In a sense, all records in the database (or all records in all tables that extend the Task table, at least) are in one monster-sized table. This means that the primary key (Sys ID) for a given record really must be globally unique.