Re: delete from jon

From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 05 Sep 2009 21:26:29 GMT
Message-ID: <9IAom.42885$Db2.14560_at_edtnps83>



paul c wrote:
...
> Delete from join where A and B have different headings is more
> complicated because other constraints as well as intermediate
> projections are involved. Regardless, the basic ideas are two: i)
> delete means delete, ie., D is asserted false and if a result that makes
> D false and violates no constraint is possible then that is the result
> and ii) unlike when A and B have equal headings, when the headings are
> unequal a delete from one doesn't necessarily result in a delete from
> the other. If the join has attributes {a, b} and A has attributes {a},
> and we try to delete the tuple <a 1, b 2>, the deletion of <a 1> from A
> doesn't imply deleting <a 1, b 2> from C and <a 1, b 2> must remain true
> ...

typo': should say "... try to delete the tuple <a 1, b 2>, the deletion of <a 1> from A doesn't imply deleting <a 1, b 1> from C and <a 1, b 1> must remain true." Received on Sat Sep 05 2009 - 16:26:29 CDT

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