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From: David BL <davidbl@iinet.net.au>
Newsgroups: comp.databases.theory
Subject: Re: more closed-world chatter
Date: 5 May 2007 08:00:25 -0700
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On May 5, 3:16 am, Marshall <marshall.spi...@gmail.com> wrote:

[snip]

> def pricedomain = set {1, 2};
> def anotherdomain = set {3};
>
> def R1 = set(price) [price : pricedomain]  {(1)};
> def R2 = set(price) [price : anotherdomain]  {(3)};
>
> def R = R1 & R2;
> ---
>
> At this point, what is the value of R?
>
>   R = set(price) [price:pricedomain; price:anotherdomain] {};
>
> Since the union of pricedomain and anotherdomain is empty:

IMO it's confusing to talk about taking a "union of the constraints".
In a sense you are aggregating them.  I would call it an ANDing of
constraints as boolean valued expressions, which leads to set
intersections not unions.