Re: Possible problems with Date & McGoveran View Updating
From: Jan Hidders <jan.hidders_at_pandora.be>
Date: Sun, 14 Sep 2003 14:59:04 GMT
Message-ID: <Ym%8b.20114$7y3.1376399_at_phobos.telenet-ops.be>
>
> This view is not updateable. If you delete a tuple from this view, do you
> delete from the base table or update dept = 'sales' into some other value
> (which we can't deduce from TrEmps alone)?
Date: Sun, 14 Sep 2003 14:59:04 GMT
Message-ID: <Ym%8b.20114$7y3.1376399_at_phobos.telenet-ops.be>
Mikito Harakiri wrote:
> "Jan Hidders" <jan.hidders_at_pandora.be> wrote in message
> news:PX38b.14807$2Z.580673_at_phobos.telenet-ops.be...
>> A small example. Suppose we have a table Emps(name,dept) and a view >> TrEmps defined by >> >> SELECT name >> FROM Emps >> WHERE dept = "sales"; >> >> then if you add a tuple to this view it is perfectly clear which tuple >> should be added to Emps and so this is a well-defined update.
>
> This view is not updateable. If you delete a tuple from this view, do you
> delete from the base table or update dept = 'sales' into some other value
> (which we can't deduce from TrEmps alone)?
According to the definition I gave this deletion is well-defined. The unique "minimal" solution is that the corresponding tuple is deleted from Emps, and indeed that is all that the database knows for certain: this tuple has to to be removed. I find this quite intuitive, can you explain why you don't?
By the way, what is exactly your definition of "updatable"? Should there be at least one instance and one update such that the result is well-defined, or should for all instances all updates be definied, or something in between?
- Jan Hidders