W3C home > Mailing lists > Public > www-webdav-dasl@w3.org > April to June 2000

RE: Order By

From: Tapas Nayak <tapasnay@Exchange.Microsoft.com>
Date: Fri, 9 Jun 2000 00:55:36 -0700
Message-ID: <00FACE8979F06F4C8B185017A14CF4E40F5838@DF-SCRAPPY.platinum.corp.microsoft.com>
To: "Babich, Alan" <ABabich@filenet.com>, "Kevin Wiggen" <wiggs@xythos.com>, <www-webdav-dasl@w3.org>
It may be restrictive to prescribe one way or the other by 
interpreting nulls with one interpretation. NULLs have 
different interpretations in different semantic contexts. 
While for strings a NULL may be considered as a NULL string, in 
many cases NULL represent values not known or dont care. For an integer
field, say age, it may be incorrect to consider NULL as 0, for example.
It is this lack of a unique concrete interpretation of NULL in all
situations 
that left its interpretation to the implementation in SQL spec, I think.

And probably that is the appropriate thing to do here too. Thus option 3
seems to be 
the appropriate thing. I understand, however, that they will pose a
problem in collation 
of results from more than one data sources, if the participating data
sources chose 
different implementations for the same property. That would indicate
possibly that 
even though it is the same property, different providers have taken
slightly different 
semantics for it. In that case either collator will have to do some
reconciliation
of these different interpretations or enforce one semantics over the
other.

Regards
Tapas
-----Original Message-----
From: Babich, Alan [mailto:ABabich@filenet.com]
Sent: Thursday, June 08, 2000 6:43 PM
To: 'Kevin Wiggen'; www-webdav-dasl@w3.org
Subject: RE: Order By


Does anybody else out there have an opinion?

The choices are:

(1) Nulls sort first
(2) Nulls sort last
(3) Nulls sort first or last at the implementation's choice

These three choices are compatible with the SQL spec., and
make logical sense. Only choice (1) is compatible with DMA.

Consider how zero length strings sort: 
"A" sorts before "AA", because it is a shorter string. 
(This is not only specified in the SQL spec., 
it is the way things are listed in the phone book.)
Therefore, the zero length string, "", sorts before "A"
and before all other strings with length greater than zero.

A null string is similar to a zero length string since
neither contains any characters, so it seems odd to me 
to put null strings and zero length strings at opposite ends 
of the sort. I would think doing that could confuse some end users 
who are not sophisticated in the subtleties of zero length 
strings versus null strings. It also forces the implementation 
to split the semantic hair of whether zero length strings are 
null or not. I would prefer not to risk confusing users or 
implementers. In other words I prefer choice (1), and I
claim that choices (2) and (3) have some risk of confusing
some end users and implementers because of the similarity of
null strings and zero length strings.

(BTW, I agree with you, Kevin, that postgresql seems to be 
violating the SQL spec. Nulls must always sort first or last, 
so DESC must put nulls at the opposite end. In postgresql, 
as you have shown, DESC doesn't do that.)

Alan

-----Original Message-----
From: Kevin Wiggen [mailto:wiggs@xythos.com]
Sent: Thursday, June 08, 2000 4:42 PM
To: Kevin Wiggen; Babich, Alan; Jim Davis; www-webdav-dasl@w3.org
Subject: RE: Order By



OK I will clarify (not that anyone cares)

Oracle

order by column1 => nulls sort last
order by column1 DESC => nulls sort first

Postgresql

order by column1 => nulls sort last
order by column1 DESC => nulls sort last

I could argue that either one makes sense to me (although postgresql
seems
to be violating the spec)....

Kevin


-----Original Message-----
From: www-webdav-dasl-request@w3.org
[mailto:www-webdav-dasl-request@w3.org]On Behalf Of Kevin Wiggen
Sent: Thursday, June 08, 2000 4:03 PM
To: Babich, Alan; Jim Davis; www-webdav-dasl@w3.org
Subject: RE: Order By




Well Oracle and Postgresql (the only db's I have around, I called a
friend
to get Informix but he's not around), both sort the nulls last.  This
makes
sense to me, as if it doesn't exist, then you probably don't care about
it,
thus it sorts last.

So I guess I object to the spec.  I would like to see it changed so that
nulls sort last.

Kevin

-----Original Message-----
From: Babich, Alan [mailto:ABabich@filenet.com]
Sent: Wednesday, June 07, 2000 8:50 PM
To: 'Kevin Wiggen'; Jim Davis; www-webdav-dasl@w3.org
Subject: RE: Order By


I just looked it up. I thought the SQL spec. required nulls to sort
first.
Here's what the version of the SQL spec. that I have actually says:

ANSI X3.135-1992 (Database Language SQL) in section 13.1, General Rule
3)b),
page 309, says "Whether a sort key value that is null is considered
greater
or
less than a non-null value is implementation-defined, but all sort key
values that
are null shall either be considered greater than all non-null values or
be
considered
less than all non-null values."

Last time I used Oracle (which was a while ago) it sorted nulls first.
To me, logically, null is even less significant than zero, so it seems
right
to me
that nulls should come first. I seem to remember that DMA query requires
nulls to sort first.

I personally think it's better for everybody to do things the same way
(that's why we have specs.,
isn't it?).

We made it definite in DASL, and made nulls sort first.
Any significant objections, or are there just comments but not a true
objection?


Alan Babich

-----Original Message-----
From: Kevin Wiggen [mailto:wiggs@xythos.com]
Sent: Wednesday, June 07, 2000 3:26 PM
To: Jim Davis; www-webdav-dasl@w3.org
Subject: RE: Order By



After further review :)

The fact that nulls sort first????, if the value is null it is probably
because the property isn't defined and therefore you could care less,
but
these are the ones sorted first???  The spec acts as though this is
common
in ANSI standard SQL, but I have only seen the opposite from every DB I
have
ever worked with (which maybe means none of them follow the standard)

Kevin


-----Original Message-----
From: Jim Davis [mailto:jrd3@alum.mit.edu]
Sent: Wednesday, June 07, 2000 7:47 AM
To: Kevin Wiggen; www-webdav-dasl@w3.org
Subject: Re: Order By


At 10:16 PM 6/6/00 -0700, Kevin Wiggen wrote:
>
>
>Does adding "Order By Content Length" (or any other file specific
property)
>automatically add "AND Resource Type = File" to the Where clause of a
Dasl
>Query?
>
>Or in English, does adding a file specific property to the orderby
clause
in
>a dasl query implicitly make all resources that are returned files?  or
do
>the directories and null resources simply sort last?

The latter.   Nowhere does  the spec say that anything is added
(implicitly) to the where clause, and I would never agree to that.  But
it
does say  "In the context of the DAV:orderby element, null values are
considered to collate before any actual (i.e., non null) value,
including
strings of zero length".  (5.6 in the most recent draft)

Is this a problem either technically or editorially?

regards

Jim
Received on Friday, 9 June 2000 04:09:13 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Sunday, 22 March 2009 03:38:05 GMT