Re: Relational Databases and Their Guts

To follow up further with my example. Let us call the act of taking any RCOL(RnPK) and making them into attributes of any new view Rx, Rotation.

1. IS Rotation definable in terms of the basic relational operators of SELECT, PROJECT, etc.
2. Since Rotation MUST introduce empty data, unless the number of elements in RCOL is the same for each RCOL for all RnPK, then:
3. Should there not be something like NULL, but does not mean undefined, but merely, empty?
4. If it is provable that there exists cases where a resulting view is not updatable, then, is not the case, that no database, regardless of implementation, can actually satisfy the updateable view requirement?
5. If in fact, the requirements for a relational database are theoretically impossible to meet, because for any implementation you can always create a non-updatable view, and, because, you have to introduce NULL to do rotation, then, isn't fair to say that no database can actually be a relational database?