| Subject | Re: [firebird-support] The use of domains |
|---|---|
| Author | unordained |
| Post date | 2004-07-23T21:14:29Z |
It's also a reference to the mathematical notion underlying relations and their attributes. A
relation is a set of points in N-space, where each dimension of the point is in a specific domain.
A domain in mathematics refers to the input range on a function -- the set of possible values for
which it is defined, and for which it has an output range. (Domain -> Range / Input -> Output). An
attribute's domain constrains the possible points in N-space which can be in that set (relation).
All attributes should have a domain (which is more like a constraint), not just an underlying
datatype. That's hidden from you by domains being named the same as the datatype, or automatically
named for you. (Ib-Expert, for example, will show you a different domain name for just about every
column in all tables, unless you've created your own domains and used them consistently. That's
because you never said you really wanted all of those attributes to be kept of the same domain,
they just happen to have been defined similarly initially. By naming domains and using named
domains, you say "the constraints on this attribute will always be the same as the constraints on
that attribute". This is particularly relevant for PK/FK fields.)
-Philip
---------- Original Message -----------
From: Daniel Rail <daniel@...>
To: firebird-support@yahoogroups.com
Sent: Fri, 23 Jul 2004 17:09:03 -0300
Subject: Re: [firebird-support] The use of domains
relation is a set of points in N-space, where each dimension of the point is in a specific domain.
A domain in mathematics refers to the input range on a function -- the set of possible values for
which it is defined, and for which it has an output range. (Domain -> Range / Input -> Output). An
attribute's domain constrains the possible points in N-space which can be in that set (relation).
All attributes should have a domain (which is more like a constraint), not just an underlying
datatype. That's hidden from you by domains being named the same as the datatype, or automatically
named for you. (Ib-Expert, for example, will show you a different domain name for just about every
column in all tables, unless you've created your own domains and used them consistently. That's
because you never said you really wanted all of those attributes to be kept of the same domain,
they just happen to have been defined similarly initially. By naming domains and using named
domains, you say "the constraints on this attribute will always be the same as the constraints on
that attribute". This is particularly relevant for PK/FK fields.)
-Philip
---------- Original Message -----------
From: Daniel Rail <daniel@...>
To: firebird-support@yahoogroups.com
Sent: Fri, 23 Jul 2004 17:09:03 -0300
Subject: Re: [firebird-support] The use of domains
> Hi,------- End of Original Message -------
>
> At July 23, 2004, 15:24, Epstein, Ed wrote:
>
> > I still dont understand why it is called a domain though.
>
> It's the SQL standard naming.
>
> --
> Best regards,
> Daniel Rail
> Senior System Engineer
> ACCRA Group Inc. (www.accra.ca)
> ACCRA Med Software Inc. (www.filopto.com)
>
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