RE: RE: Useful Oracle books - C.J. Date theory vs. practicality

From: Lex de Haan <lex.de.haan_at_naturaljoin.nl>
Date: Fri, 28 May 2004 15:28:10 +0200
Message-ID: <JFEEIGBIDOCCDALDIPLNIEOFCDAA.lex.de.haan@naturaljoin.nl>

Hi Ryan,

the definition of a domain:
a set of allowed values, with a set of constraints and well-defined operators on those values.
in other words, it is "just" a data type. however, this entails a little bit more than
the standard NUMBER, VARCHAR, DATE data types...

the ANSI/ISO SQL standard has the following syntax:

<domain definition> ::=
CREATE DOMAIN <domain name> [ AS ] <predefined type>
[ <default clause> ]
[ <domain constraint>... ]
[ <collate clause> ]

<domain constraint> ::=
[ <constraint name definition> ] <check constraint definition> [
<constraint characteristics> ]

by the way, this is not part of core SQL; it is part of feature F251, "Domain support"

user-defined datatypes and methods come sort of close, by the way.

your second question is tough -- can't answer that in a line or two. three-valued logic is already rather counter-intuitive for human beings, and SQL did not implement it in a consistent way. For example:

compare the implementations of [NOT] IN and [NOT] EXISTS
see what group functions do with null-values
consider under which conditions SQL is returning null-values, even when your database doesn't contain a single null-value...

Kind regards,
Lex.

visit my website at http://www.naturaljoin.nl <http://www.naturaljoin.nl>

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of ryan.gaffuri_at_cox.net Sent: Friday, May 28, 2004 13:35
To: oracle-l_at_freelists.org; oracle-l_at_freelists.org Subject: Re: RE: Useful Oracle books - C.J. Date theory vs. practicality

Lex,

For those of us not schooled in relational theory could you explain what you mean by 'domain'?

Also what are your issues with SQL?

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Received on Fri May 28 2004 - 08:25:38 CDT