Re: 9/18 Visual Planner Meeting Wrapup

From: "Len Shapiro" <lenshap(at)gmail(dot)com>
To: "David E(dot) Wheeler" <david(at)kineticode(dot)com>
Cc: "Mark Wong" <markwkm(at)gmail(dot)com>, "Postgresql PDX_Users" <pdxpug(at)postgresql(dot)org>, "David Maier" <maier(at)cs(dot)pdx(dot)edu>
Subject: Re: 9/18 Visual Planner Meeting Wrapup
Date: 2008-10-21 14:40:58
Message-ID: c5ee9b8a0810210740s6a52c763ie703c3c331f6da0e@mail.gmail.com
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David,

Thanks for the astute observation. By "foreign key join" I did not
mean just a join between foreign keys. It is difficult to compute the
cardinality of such a join. I meant a join between a foreign key and
the (unique) attribute to which it refers. It is simple to compute
the cardinality of this latter type of join.

All the best,

Len

On Mon, Oct 20, 2008 at 9:29 PM, David E. Wheeler <david(at)kineticode(dot)com> wrote:
> On Oct 20, 2008, at 20:38, Len Shapiro wrote:
>
>> "Who are the congressional candidates running in my district?"
>>
>> SELECT candname
>> FROM candidate JOIN resident ON candidate.zip = resident.zip
>> WHERE resident.name = 'Len Shapiro'
>
> And would that be a join that's not on a foreign key because candidate.zip
> and resident.zip are foreign keys to a zip_codes table, and not otherwise
> related to one another?
>
> Best,
>
> David
>

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