Jan's well-defined view updates definition

"Jan Hidders" <jan.hidders_at_pandora.be> wrote in message news:jWo9b.22053$NH3.1410149_at_phobos.telenet-ops.be...
> >> No, this is an illegal update because you are trying to insert a tuple
> >> into
> >> a relation that cannot have such a tuple. This makes sense because your
> >> view is your ontology so the only thing that you can tell the database
is
> >> that someone is no longer with sales. It's the user with the
> >> marketing-view
> >> that should tell the database that this employee is now with marketing.

You don't have any constraint on the base table. Therefore,

deltaD = (
 update Emps
 set dept = 'marketing'
 where dept = 'sales'
)

is legal. This update is "projected" into the view but the projection looses some information. For example,

deltaD = (
 delete from Emps
 where dept = 'sales'
)

would be indistinguisheable from the above update. You suggest restoring the lost information by accepting the second update as "natural" translation of

deltaV = (
 delete from TrEmps
)

My position is that we restore missing information by considering more views. For example, suppose we have

Q1 = (SELECT name
 FROM Emps
 WHERE dept = "sales"
)
Q2 = (SELECT name
 FROM Emps
 WHERE dept <> "sales"
)

and

deltaV1 = (
 delete from TrEmps
)

(It might be a good idea to switch notation here from TrEmps to V1)

deltaV2 = no change

Then

Q * (delete from Emps where dept = "sales") = = <deltaV1,deltaV2>

(Here we have multiple relations on both sides of the equation, therefore I swithed to notation that uses "vector" of deltas).

We see that Q is updateable for that particular combination of deltas, but it wouldn't be updateable for

deltaV1 = no change

deltaV2 = (
  delete from V2
)

Hence, "local invertibility".

> That doesn't matter, the definition would still define a trivial property.
> As I already said, you can always take for Q^-1 the function that maps any
> relation (or any tuple, if you change the definition) to X.

Answered in the old thread. Received on Mon Sep 15 2003 - 15:47:08 CDT