Re: More on view updates and inverse views

From: Mr. Scott <do_not_reply_at_noone.com>
Date: Thu, 10 Sep 2009 04:38:28 -0400
Message-ID: <t-OdnUOzd8GZJjXXnZ2dnUVZ_uCdnZ2d_at_giganews.com>


"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> Received on Thu Sep 10 2009 - 03:38:28 CDT

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