Re: more on delete from join

From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 23 Aug 2009 21:35:43 GMT
Message-ID: <PCikm.40415$Db2.8889_at_edtnps83>



paul c wrote:
> paul c wrote:
>> ...
>> A' = (J MINUS D} {hA} UNION (A MINUS J{hA})

>
> (because J MINUS D = J' MINUS D.)
>

The reason I mentioned this is because if the algebraic result, according to the equations I gave, is

J:
a b
1 3
1 4
2 3
2 4

then J MINUS D = J' MINUS D becomes a contradiction. There might be other ways for a language to treat that assertion, but I would say the more clear-cut one is to treat it as what it is, a constraint. Since the algebraic result breaks the constraint, the result is more likely an empty J value. Received on Sun Aug 23 2009 - 16:35:43 CDT

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