Re: delete cascade
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 17 Apr 2007 13:25:23 GMT
Message-ID: <7V3Vh.25374$PV3.257551_at_ursa-nb00s0.nbnet.nb.ca>
>
>
> Sorry for not explaining that.
>
>
>
> Well, if I try to delete invoice 99 while there are items for that
> invoice the result of the "OR" would not include invoice 99 and so would
> be empty as far as invoice 99 is concerned and so would not satisfy the
> constraint. (I realize I'm using a generalized form of "OR" here which
> is why I might prefer to express the constraint using "AND".)
Date: Tue, 17 Apr 2007 13:25:23 GMT
Message-ID: <7V3Vh.25374$PV3.257551_at_ursa-nb00s0.nbnet.nb.ca>
paul c wrote:
> Bob Badour wrote:
>
>> 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. >> ...
>
>
> Sorry for not explaining that.
>
>>
>>> -> 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?
>
>
> Well, if I try to delete invoice 99 while there are items for that
> invoice the result of the "OR" would not include invoice 99 and so would
> be empty as far as invoice 99 is concerned and so would not satisfy the
> constraint. (I realize I'm using a generalized form of "OR" here which
> is why I might prefer to express the constraint using "AND".)
I understand how it expresses the constraint. What does that have to do with cascading, though? Received on Tue Apr 17 2007 - 15:25:23 CEST