W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > April to June 2004

Indulgent SQL diversion

From: Eric Prud'hommeaux <eric@w3.org>
Date: Fri, 11 Jun 2004 16:43:17 +0900
To: Jim Hendler <hendler@cs.umd.edu>
Cc: Rob Shearer <Rob.Shearer@networkinference.com>, RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-ID: <20040611074317.GA15900@w3.org>

On Wed, Jun 09, 2004 at 08:27:12PM -0400, Jim Hendler wrote:
> 
> At 16:59 -0700 6/9/04, Rob Shearer wrote:
> >> In my experience most query languages don't actually handle
> >> disjunctive queries very well (SQL, for example, only allows the OR
> >> in the conditional of a return, which is a very limited kind of OR).
> >
> >I don't entirely understand this, and I'm curious. I had always rather
> >assumed that WHERE clauses could contain arbitrary boolean combinations
> >of predicates. Am I misunderstanding you, or am I misunderstanding SQL?
> 
> the definition of "conditional of a return" is in a WHERE clause. 
> But my understanding of SQL is that you cannot make arbitrary query 
> clauses -- you get the binding list and then you process the results 
> against the WHERE clause -- i.e. you can say
>  Fetch (complex expression, but no disjunction includign A, B, C, etc)
>    WHERE
>      A = "foo" OR B = "bar" ...

I think this is called "underconstrained disjunction" by SQL folks.

You can ask the question that I suspect you mean, but you have to
teach the server how to do that kind of "OR" in terms of relational
calculus. Starting with the explicit form of JOINs:

SELECT stuff
FROM table1
     INNER JOIN table2 ON table1.A=table2.foo
     INNER JOIN table2 ON table1.B=table3.bar

you get A=foo AND B=bar. When you want to change that to OR, the
tempation is:

SELECT stuff
FROM table1
     INNER JOIN table2 ON table1.A=table2.foo OR 1
     INNER JOIN table2 ON table1.B=table3.bar OR 1

which gives you the AND results plus table1 X table2 X table3. The
issue is that you don't want to eliminate a row from table1 just
because it doesn't line up with table2 (it may match the other
side of the disjunction). So:

SELECT *
FROM table1
     LEFT OUTER JOIN table2 ON A=foo
     LEFT OUTER JOIN table3 ON B=bar
WHERE A=foo OR B=bar;

works. If the server executes the query recursively, you won't incur
any more cost with the OUTER JOINs than you would with any other query
engine handling the A=foo OR B=bar constriant.


BTW, I checked these examples. see below:

CREATE TABLE table1 (A CHAR(20), B CHAR(20));
CREATE TABLE table2 (foo CHAR(20));
CREATE TABLE table3 (bar CHAR(20));
INSERT INTO table1 (A,B) VALUES ('A0', 'B0');
INSERT INTO table1 (A,B) VALUES ('A1', 'B1');
INSERT INTO table1 (A,B) VALUES ('A2', 'B2');
INSERT INTO table1 (A,B) VALUES ('A3', 'B3');
INSERT INTO table1 (A,B) VALUES ('A6', 'B6');
INSERT INTO table2 (foo) VALUES ('A0');
INSERT INTO table2 (foo) VALUES ('A2');
INSERT INTO table2 (foo) VALUES ('A4');
INSERT INTO table2 (foo) VALUES ('A6');
INSERT INTO table3 (bar) VALUES ('B0');
INSERT INTO table3 (bar) VALUES ('B3');
INSERT INTO table3 (bar) VALUES ('B6');

SELECT *
FROM table1
     INNER JOIN table2 ON A=foo
     INNER JOIN table3 ON B=bar;

SELECT *
FROM table1
     LEFT OUTER JOIN table2 ON A=foo
     LEFT OUTER JOIN table3 ON B=bar
WHERE A=foo OR B=bar;

-- 
-eric

office: +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA
cell:   +1.857.222.5741

(eric@w3.org)
Feel free to forward this message to any list for any purpose other than
email address distribution.
Received on Friday, 11 June 2004 03:43:09 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:15:19 GMT