W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > April to June 2005

[Fwd: Comments on ORDER BY]

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Thu, 26 May 2005 10:49:07 +0100
Message-ID: <42959B93.8090004@hp.com>
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>

This is a comment that I forgot to process fully.  This points out that ORDER BY 
is restrictive in that it only provided for a variable or a function.

http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2005May/0005.html

Richard Cyganiak wrote:
> Comments on 
> http://www.w3.org/2001/sw/DataAccess/rq23/
> regarding the ORDER BY clause:
> 
> 
>>From the grammar:
> 
> | [16] OrderExpression ::= FunctionCall | Var
> | [59] FunctionCall ::= URI '(' ArgList ')'
> 
> This doesn't allow expressions like "?a + ?b" in the ORDER BY clause. Is
> this intentional?
> 
> If not, this sentence from section 10.1 also needs updating:
> 
> | An ordering condition can be a variable or a function call.


There is no reason I can see not to allow an expression but the grammar need 
tweaking.

We have the allowance for a fucntion because of the need to handle 
domain-specific datatypes.

But we can't just put "Expression" at this point because this is ambigous: + and 
- can occur both as unary and binary operators.  We allow multiple ordering 
conditions on a single line so

ORDER BY ?a + ?b

It's either two ordering conditions:

ORDER BY ?a
ORDER BY +?b

or it's one condition

ORDER BY (?a+?b)

We already have grouping using ASC/DESC.  We choose [] (mildly) because ASC/DESC 
aren't functions.  We (for consistency) use blank separated lists of conditions.

0/ No change.  Don't allow general expressions.

1/ change from using [] to using () for ASC and DESC

ORDER BY (?a+?b)
ORDER BY ASC(?a+?b)

2/ Make special cases for () expressions inside and outside [].

ORDER BY (?a+?b)
ORDER BY ASC[?a+?b]

3/ Always require () on expressions which are not simple variables or function 
calls:

ORDER BY (?a+?b)
ORDER BY ASC[(?a+?b)]

4/ Only one condition per ORDER BY; multiple ORDER BY clauses.


I propose #0 :
For a change to support expressions, #1 is the best in my opinion.

#3 is downright ugly.

Context:

Standard SQL does not support expressions but some system do as an extension:
e.g. http://www.postgresql.org/docs/8.0/static/queries-order.html

MySQL 5.0 does not seem to mention it.

	Andy

<snip class="stuff that has been dealt with"/>
Received on Thursday, 26 May 2005 09:49:21 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:15:23 GMT