Re: A different definition of MINUS, part 4

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 09 Jan 2009 13:42:03 -0800
Message-ID: <bvP9l.2931$2w3.1859_at_newsfe19.iad>



Bob Badour wrote:
...
> As I said, I am unsure of the notation. Above, I more-or-less assumed
> that GROUP would create a relation valued attribute from some specified
> set of attributes while projecting on the remaining attributes, but one
> probably wants to group things other than attributes in a single
> relation, and one probably wants the option to project on an arbitrary
> set of attributes.
>
> Ignoring those notation problems, though, I assume you see how that
> more-or-less expresses "Suppliers who supply all parts."
>
> One could replace P{P} with any expression answering "All of what?" For
> example, "Suppliers who supply all the green parts." might be:
>
> (SP{S,P} GROUP Parts{P} | Parts = (P | COLOR='green'){P}){S}
>
> assuming the P relation has a COLOR attribute and one of the possible
> values is 'green'.
> ...

Yes, I can see `all parts` and `all green parts`, as long as there are some green parts. If there aren`t any green parts, I`d say we get the same answer as Codd`s Divide, in which all suppliers might not appear. Is that why you said "one probably wants to group things other than attributes in a single relation"?

> I used the vertical bar for RESTRICT similar to set notation, which I
> generally read as "such that", and {} for PROJECT. Although, I have to
> say that as my eyes age, I like notations that use both () and {} less
> and less.

I'm okay with the bar, took it as a kind of cumulative <AND>, probably saves a lot of fiddly typing to express conversions. Received on Fri Jan 09 2009 - 15:42:03 CST

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