Re: c.d.theory glossary - domain

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Fri, 14 May 2004 22:37:37 +0200
Message-ID: <40a52e15$0$566$e4fe514c@news.xs4all.nl>

Paul wrote:

> Dawn M. Wolthuis wrote:

>>>> [Domain]
>>>> Given a relation R, a domain is a set Sn such that for each tuple (A1,
>>>> A2, ...An, ...Am) in R, An is an element of Sn.
>>
>> That's a mathematical def of domain, which is where Codd started.  
>> Again, a
>> "b" definition might be in order -- what do you like as a def?

>
> Surely domain is a lower-level concept than relation. So it doesn't make
> sense to refer to relations in the initial definition of what a domain
> is. I think having a few examples is also very helpful.
>
> I'd say something like:
>
> A "domain" is a set of values: for example "integers between 0 and 255",
> "character strings less than 10 characters long", "dates". Sometimes
> used synonymously with "type".

Happy to add this.

> In a relational database context, each column of a table is constrained
> to only allow values from a specific domain. For example, this helps to
> ensure that a "name" won't get accidently stored in a "birthdate" column.

I think the glossary should not mix terms like tables/relations and column/attributes this way. To many people these have very different connotations. But I'll admit I do mix them myself, occasionally.

>
> ---
> Sometimes a "domain" is taken to include operators on the set, as well
> as the set of values itself. For example an integer domain might include
> addition and multiplication operators, and greater than / less than
> operators.
>
> I'm not sure about this last bit though because what about operators
> that take arguments from more than one domain? Or operators that return
> a value that isn't in the domain of the arguments? Where do they fit in?

[Todo]
+ Operator Received on Fri May 14 2004 - 15:37:37 CDT