Hi,
I am trying to run some SPARQL queries translated to SQL on NATIVE Virtuoso
columnstore, which involve combination of left join and UNION statements.
A sample SPARQL query is given below:
*Q1:*
SPARQL SELECT * FROM <http://mytest.com>
WHERE {
?a <http://ex.com/11> <http://ex.com/1> .
OPTIONAL {
{?a <http://ex.com/11> <http://ex.com/1> .
OPTIONAL {?a <http://ex.com/12> ?b .}
}
UNION
{?a <http://ex.com/11> <http://ex.com/1> .
OPTIONAL {?b <http://ex.com/11> ?a .}
}
}
}
Now, to run this as an SQL query, I stored the same data from the graph <
http://mytest.com> in a 3-column table called "mytest", and translated the
above query to SQL as below:
*Q2:*SELECT * FROM
(select s as a from mytest where p=11 and o=1) as t1
left join
(select t2.s as a, t3.o as b from
(select s from mytest where p=11 and o=1) as t2
left join
(select s, o from mytest where p=12) as t3
on t2.s=t3.s
UNION ALL
select t4.s as a, t5.s as b from
(select s from mytest where p=11 and o=1) as t4
left join
(select s, o from mytest where p=11) as t5
on t4.s=t5.o) t7
on t1.a=t7.a;
*The problem is* -- while Q1 runs fine on Virtuoso, Q2 keeps giving me
syntax a error at the very last line (on t1.a=t7.a). I checked Virtuoso's
SQL grammar at http://docs.openlinksw.com/virtuoso/SELECTSTMT.html as well
as looked at the abstract representation of Q1 through "set explain on".
But neither of this has helped me to know where I am going wrong.
Any other SQL query with only left joins, without UNION statement run
correctly, but as soon as do a left join over a UNION ALL statement, it
gives a syntax error. To debug more, I tried running just the subquery of
Q2 having only UNION ALL as
*Q3:*
select t2.s as a, t3.o as b from
(select s from mytest where p=11 and o=1) as t2
left join
(select s, o from mytest where p=12) as t3
on t2.s=t3.s
UNION ALL
select t4.s as a, t5.s as b from
(select s from mytest where p=11 and o=1) as t4
left join
(select s, o from mytest where p=11) as t5
on t4.s=t5.o) t7
*Q3 runs correctly, but Q2 refuses to run by giving syntax error.*
Can the Virtuoso support staff or the users let me know how to correctly
write Q2 to make it run on the native SQL engine? *Please note* that I have
to run the SQL format of the query to do some native SPARQL vs SQL
experiments, so solutions like "just run the SPARQL query only" will not be
useful!
Thanks in anticipation.
Medha
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