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Re: delete cascade

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 17 Apr 2007 13:12:30 GMT
Message-ID: <2J3Vh.25372$PV3.257299@ursa-nb00s0.nbnet.nb.ca>


paul c wrote:

> Bob Badour wrote:
>

>> paul c wrote:

>
> ...
>
>>> Sorry, I think I put that wrongly.  Maybe the constraint that "one 
>>> may not delete an invoice when any items exists would look something 
>>> like "(NOT Invoices{Invoice#}) AND Items{Invoice#} = FALSE", ie., a 
>>> reference from the complement of invoices to items.
>>
>> I am not sure I fully understand your syntax and the order of 
>> precedence you are using, but wouldn't demorgan make that:
>>
>> Invoices{Invoice#} OR Items{Invoice#}
>>
>> That doesn't seem like the constraint at all to me.

>
> Not sure I entirely understand my notation either but here's what I get
> if I substitute "A" for the invoices projection and "B" for the items
> projection:
>
> (not A) and B = false /* ie., the join is empty */
>
> -> not ((not A) and B) = true /* negate both sides */

Ahhh, so 'and' and 'or' have higher precedence than '='. That explains much.

> -> A or (not B) = true /* de Morgan */
>
> in other words,
> Invoices{Invoice#} OR NOT(Items{Invoice#}) is not empty.

Okay, that makes sense. How does it relate to cascading again? Received on Tue Apr 17 2007 - 08:12:30 CDT

Original text of this message

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