Re: More on view updates and inverse views

From: paul c <toledobythesea_at_oohay.ac>
Date: Thu, 10 Sep 2009 23:31:24 GMT
Message-ID: <g%fqm.44485$PH1.11973_at_edtnps82>



Mr. Scott wrote:
> "Vadim Tropashko" <vadimtro_at_gmail.com> wrote in message
> news:dde2aed2-8d1f-4793-b481-a435383934e4_at_m7g2000prd.googlegroups.com...
>> On Sep 8, 9:35 am, "Walter Mitty" <wami..._at_verizon.net> wrote:
>>> This makes the projection view
>>> "invertable"  even though without the default value it would not be
>>> invertable.
>> And what is default value in relational terms?

>
> I think that depends on how a default constraint is expressed logically.
>
> For a table (x,y,z) that neither allows nulls nor has default constraints,
> the predicate could be Pxyz. If the table were to have a default constraint
> for y, what would the logical expression for the table become? Isn't it
>
> (Pxyz xor Pxdz) or (Pxyz and (y = d))?
>
> If it is, then I think a view over (x,z) should be updatable.
>
> <snip>
>
>

My attitude is that if it can't be expressed in an algebra it shouldn't be implemented. Not saying there isn't a way, but I don't know how to express defaults as a constraint nor algebraically. Seems to me a union is always involved, perhaps an exclusive one as you suggest, but the main thing is that I don't know how to do it without declaring a base relvar with attributes x and z, (so it wouldn't be a view). I'd be happy to be corrected! Received on Thu Sep 10 2009 - 18:31:24 CDT

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