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Re: relations aren't types?

From: Jonathan Leffler <jleffler_at_earthlink.net>
Date: Tue, 30 Dec 2003 07:41:37 GMT
Message-ID: <R_9Ib.11658$lo3.10578@newsread2.news.pas.earthlink.net>


Marshall Spight wrote:

> "Lauri Pietarinen" <lauri.pietarinen_at_atbusiness.com> wrote:

>> My understanding is that in the scope of relational databases, 
>> atomicity is defined in terms of whether the relational operators
>> can "see" the value or not without the help of "non-relational",
>> or scalar operators.

>
> Interesting. I wonder if that's definition Bob is using, thus causing
> our disconnect about atomicity. Where did you get this definition,
> may I ask?

Obviously, I can't answer for Lauri, but...

It strikes me as being a good statement of an atomic type, and one that can be derived from the writings of Date (and Darwen). I've not seen it in quite that form before - which might mean Lauri has found a new way of stating what atomicity means, or it might mean I've not read the relevant book or paper.

One point that has often bugged me is the assumption that arithmetic is part of the relational model. It's usually expressed in terms of 'but of course a real DBMS would provide an arithmetic capability'. Binary infix operators such as '+' and '||' (for addition and string concatenation respectively) are not relational; they are non-relational scalar operators defined on some types. In the case of '+', it is polymorphic in that if you distinguish between integers and floating point numbers, then there are usually at least two versions of the operator - one for two integer values and one for two floating point values, plus usually a conversion operator that converts an integer to a floating point value when the types of the operands differ. Even equality is a non-relational operator; it takes two values (nominally of the same type, or two values coerced to the same type) and returns a boolean, and the relational model uses that boolean result. One (probably obscure and not necessarily valuable) way of looking at it is that the only type that the model must support is the boolean type -- all the others are optional.

-- 
Jonathan Leffler                   #include <disclaimer.h>
Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com
Guardian of DBD::Informix v2003.04 -- http://dbi.perl.org/
Received on Tue Dec 30 2003 - 01:41:37 CST

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