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Re: Relation Schemata vs. Relation Variables

From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Fri, 01 Sep 2006 21:43:45 +0200
Message-ID: <eda2ld$1ra$1@orkan.itea.ntnu.no>


Bob Badour wrote:
> Jon Heggland wrote:

>> vc wrote:
>>> My point is that if 'update' is shorthand for the respective relational
>>> assignment,  then by the same token the update constraint must be
>>> shorthand for the respective relvar constraint: IS_EMPTY(R RENAME
>>> (Salary AS NewSalary) JOIN R' RENAME (Salary AS
>>> OldSalary) WHERE NewSalary < OldSalary).
>>
>> In that case, the shorthand is insufficient, because it doesn't specify
>> what you should join on. You could say it should join on the key by
>> default, but what if there's more than one key? What if you want keys to
>> be able to change?

>
> Natural join joins on all attributes with the same name. Given the
> rename operations specified, that would be every attribute except
> Salary. If one wanted to limit the join, one could rename additional
> attributes or project the relations onto the attributes of interest.

Not a bad idea, but you can't really apply it to Dataphor's TCs without changing the syntax quite a bit by involving explicit renames and/or projections. The resulting shorthand may not be all that short.

-- 
Jon
Received on Fri Sep 01 2006 - 14:43:45 CDT

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