Re: more on delete from join

From: paul c <>
Date: Tue, 25 Aug 2009 15:01:08 GMT
Message-ID: <U0Tkm.40612$Db2.23627_at_edtnps83>

Mr. Scott wrote:
> "paul c" <> wrote in message
> news:IPWjm.40275$Db2.30224_at_edtnps83...

>> If A' and B' stand for the resulting values of the base relvars,
>> A' = (J MINUS D} {hA} UNION (A MINUS J{hA})
>> B' = {J MINUS D} {hB} UNION (B MINUS J{hB}).

> I don't think this is right. It assumes that a delete from J translates
> into a delete from both A and B, when a delete from either A or B would
> suffice. The predicate of a join view is the conjunction of the predicates
> of the tables being joined. For there to be a row in J requires that there
> be a row in both A and B, so the fact represented by a row in J is the
> conjunction of the facts represented by the corresponding rows in A and B.
> Denial of at least one of the facts represented in A or B denies the fact
> represented in J; conversely, denial of the fact represented in J denies at
> least one of the facts represented in A or B, but not necessarily both.
> ...

Yes, it does make that assumption, consequence of logical independence I think. Received on Tue Aug 25 2009 - 17:01:08 CEST

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