Re: RM and definition of relations/tuples
From: Martin Zapf <Martin_Zapf_at_gmx.net>
Date: Fri, 25 Nov 2005 11:47:51 +0100
Message-ID: <dm6q50$5i3$1_at_online.de>
>
>
> out of curiosity does this first definition originate from? Definition
> 2 certainly seems on the money to me, but in the first definition I see
> no mapping from value to attribute - this would seem essential to me
> given such a mathematical formalisation (attributes can't be
> automagically known externally to a system which presupposes that they
> may vary?).
>
> All best, J.
>
Date: Fri, 25 Nov 2005 11:47:51 +0100
Message-ID: <dm6q50$5i3$1_at_online.de>
JOG wrote:
> Martin Zapf wrote:
>
>>1. defintion: >> >> A relation r for schema R is a mathematical relation (cartesian >> product) over the >> domains from the attributes of R. >> So r:=dom(A_1)xdom(A_2)x...xdom(A_n) >> A tuple is an element of r. >>
>
>
> out of curiosity does this first definition originate from? Definition
> 2 certainly seems on the money to me, but in the first definition I see
> no mapping from value to attribute - this would seem essential to me
> given such a mathematical formalisation (attributes can't be
> automagically known externally to a system which presupposes that they
> may vary?).
>
> All best, J.
>
You can find the first definition in various books, for example in 'Database System Concepts, Silberschatz' or 'Fundamentals of Database systems, Elmasri'.
In the first definition is a littel typo: relation r should be a subset of the cartesian product dom(A_1)xdom(A_2)x...xdom(A_n).
To map the values of one tupel (element) of relation r to the attributes
you have to consider the order of the attributes in the cartesian
product which r is a subset from.
I also must admit that this definition is not satisfying for me.
Received on Fri Nov 25 2005 - 11:47:51 CET