Re: is pivoted phones view updateable?

"Aloha Kakuikanu" <aloha.kakuikanu_at_yahoo.com> wrote in news:1163713401.759925.80280_at_b28g2000cwb.googlegroups.com:

>
> NENASHI, Tegiri wrote:

>> "Vadim Tropashko" <vadimtro_invalid_at_yahoo.com> wrote in
>> news:1163183196.730989.262270_at_m73g2000cwd.googlegroups.com:
>>
>>
>> > This view is exactly the opposite of oldcontactview. While
>> > oldcontactview defines what data must be in the old table if we
>> > know the content of the new table, the newcontactview defines new
>> > table data if we know the old one. Yes, this view is definitely
>> > useful, especially if we want to migrate the data from old table to
>> > new one, but I have more advanced application in mind. I reserve
>> > new name for it - let's call it an inverse view, while the original
>> > oldcontactview would simply be a direct view. Formally, an inverse
>> > view is such a view that applying it after direct view will restore
>> > the content of the base table that the direct view uses in its
>> > declaration. 
>>
>> Let r1 = {'x' = x|x = 1..100}., v1 = {r1|x = 1..10}.  One can not
>> recover r1 from v1,  it means v1 does not have the inverse.  It means
>> that a random view is not invertible in general.

If a random view is not invertible they one must define in precise fashion what vies are are invertible.

> In your example, what is 'x'? Have you meant

'x' is the name of the attribute.

>
> r1(x)
>
> a relation with one argument x, such that domain of x is 1..100? This
> is a constraint:

r1 contains the set of 1 .. 100.

>
> r1 union {(x=1),(x=2),...,(x=100)} = r1.

That is the same that I wrote in the set builder fashion.

>
> Then, what is v1? Perhaps
>
> v1 = r1 join a1
>
> where a1={(x=1),(x=2),...,(x=10)} ?

v1 is the view that is a subset of r1: v1 = {1,2,.., 10}. I removed the attribute name for simplicity.

>
> I agree that this example is not invertible. But switch the roles of
> 10 and 100 and it becomes updateable!

Please explain.

>
>
Received on Fri Nov 17 2006 - 16:20:19 CET