Re: Relation Schemata vs. Relation Variables
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 01 Sep 2006 15:42:49 GMT
Message-ID: <ZxYJg.6981$9u.91172_at_ursa-nb00s0.nbnet.nb.ca>
>
> 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?
Date: Fri, 01 Sep 2006 15:42:49 GMT
Message-ID: <ZxYJg.6981$9u.91172_at_ursa-nb00s0.nbnet.nb.ca>
Jon Heggland wrote:
> vc wrote:
>
>>Jon Heggland wrote:
>>
[snip]
>>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. Received on Fri Sep 01 2006 - 17:42:49 CEST