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$aG1.39278_at_pd7urf3no>
>
>
> It's not a situation where division could/should be used. There are
> many ways to answer the question. One might be just to subtract those
> who did not pay from all the customers:
>
> Invoice: <invoice_id, customer_id, paid>
>
> Project(Invoice, customer_id) MINUS Project_customer_id(Select(Invoice,
> paid = false, customer_id))
>
> vj
>
Date: Fri, 27 Apr 2007 00:22:20 GMT
Message-ID: <0nbYh.133230$aG1.39278_at_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 Fri Apr 27 2007 - 02:22:20 CEST