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Re: question on UNION

From: Steve Harris <swh@ecs.soton.ac.uk>
Date: Tue, 5 Jun 2007 08:18:18 +0100
Message-Id: <8462BEE6-4F47-40B8-A12C-087006A2E76B@ecs.soton.ac.uk>
Cc: public-sparql-dev@w3.org
To: "Andrew Newman" <andrewfnewman@gmail.com>


On 5 Jun 2007, at 07:32, Andrew Newman wrote:

>
> On 6/5/07, Steve Harris <swh@ecs.soton.ac.uk> wrote:
>> On 5 Jun 2007, at 06:55, Andrew Newman wrote:
>> > On 6/5/07, Steve Harris <swh@ecs.soton.ac.uk> wrote:
>> > Isn't it just outer union?  And isn't outer union just part of  
>> SQL 92.
>> > And isn't SQL 92 implemented by most (all?) databases.
>>
>> In a sense. You can expand any SPARQL UNION into a set of SQL UNIONs,
>> but SQLs UNION doesn't allow you to explicitly write
>>     :x :y ?z { ?z :p ?q } UNION { ?z :r ?q }
>> where the ?z-s are scoped to the whole expression and the ?q-s are
>> scoped to the block. In relational the equivalent would be to join
>> two expressions, each of two joins and use rho to unify the
>> variables. But, I personally wouldn't write it that way.
>>
>
> So OUTER JOIN is a set operation not a join operation - so they are
> differently scoped in SQL too (I think).

Yes, well put, but that was my point: you can't have some variables  
that are scoped and some that are not in SQLs UNION, you can in SPARQLs.

- Steve
Received on Tuesday, 5 June 2007 07:20:24 UTC

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