Re: Why relational division is so uncommon?

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 27 Apr 2007 00:22:20 GMT
Message-ID: <0nbYh.133230$aG1.39278@pd7urf3no>

V.J. Kumar wrote:
> paul c <toledobythesea_at_oohay.ac> wrote in
> news:zpaYh.131992$6m4.9100_at_pd7urf1no:
>
>

>>Vadim Tropashko wrote:
>>...
>>
>>>>I notice also that the example doesn't suggest a lot of activity,
>>>>other than bureaucratic activity, this company might go out of
>>>>business soon if there is no "sales" or "orders" or "inventory"
>>>>relations.  Lots of other useful queries would then arise, "who has
>>>>paid all their invoices on time?", "which students have passed all
>>>>course exams?". 
>>>
>>>
>>>Quite an interesting perspective. Although, the query "who has paid
>>>all their invoices
>>>on time?" doesn't seem to be a relational division (what relation are
>>>you dividing into?)
>>>...
>>
>>Customers?
>>
>>p
>>

>
>
> It's not a situation where division could/should be used. There are
> many ways to answer the question. One might be just to subtract those
> who did not pay from all the customers:
>
> Invoice: <invoice_id, customer_id, paid>
>
> Project(Invoice, customer_id) MINUS Project_customer_id(Select(Invoice,
> paid = false, customer_id))
>
> vj
>

What if Invoice had no "paid" attribute?

p Received on Thu Apr 26 2007 - 19:22:20 CDT