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Re: ordering of primary key columsn

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 15 Aug 2012 13:46:35 +0100
Cc: public-rdb2rdf-wg@w3.org
Message-Id: <3E3E4831-C8B8-4A55-8521-3ED85A04B315@cyganiak.de>
To: Eric Prud'hommeaux <eric@w3.org>
Eric,

In the SQL model, columns in all table constrains (unique, primary key, foreign key) are ordered. This isn't terribly explicit throughout the SQL spec, but the existence of an ORDINAL_POSITION column in the KEY_COLUMN_USAGE table in the INFORMATION_SCHEMA makes it very clear. The description of this column in SQL 2008 reads:

“The value of ORDINAL_POSITION is the ordinal position of the specific column in the constraint being described. If the constraint described is a key of cardinality 1 (one), then the value of ORDINAL_POSITION is always 1 (one).”

One could add an informative note to the DM spec along these lines:

[[
Note: The order of a primary key constraint's columns is determined by the DDL statement used to create it. In SQL implementations that support the information schema, this order can be accessed through the INFORMATION_SCHEMA.KEY_COLUMN_USAGE.ORDINAL_POSITION column.
]]

Best,
Richard


On 15 Aug 2012, at 08:47, Eric Prud'hommeaux wrote:

> * Paul Tyson <phtyson@sbcglobal.net> [2012-08-15 21:14-0500]
>> Regarding http://www.w3.org/TR/2012/PR-rdb-direct-mapping-20120814/.
>> 
>> The definition of the row node IRI in section 3 says "for each column in
>> the primary key, in order...".
>> 
>> Is there a canonical ordering for primary key constraints? The examples
>> appear to follow the order of PK declaration in a DDL clause.
> 
> I *think* the answer to this is that a primary key is a candidate key (with an extran no-NULLs constraint) and candidate keys, by virtue of being referents of foreign keys, have order. That's a bit loose, e.g. who's to say that an unreferenced primary key isn't just a set? The grammar says that "PRIMARY KEY" is followed by a "<column name list>", but elsewhere referes to keys as sets. Unique constraints are defined in terms of "SELECT UCL..." where UCL is the above "<column name list>" and SELECTs are ordered. Is this sufficient grounds to believe that primary keys are ordered?
> 
> 
>> Perhaps the definition should clarify the intended order of PK column
>> names in the row node IRI. If such an order cannot be reliably derived
>> from database metadata, maybe lexical ordering could be prescribed.
> 
> I've always seen it preserved, but I haven't found that prescribed yet.
> 
> 
>> Also, in the row node definition, the last 3rd-level bullet says "if it
>> is not the last column in the foreign key,...". Perhaps that should be
>> "...in the primary key,..."?
> 
> I took care of this.
> 
> 
>> Regards,
>> --Paul
> -- 
> -ericP
> 
Received on Wednesday, 15 August 2012 12:47:00 UTC

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