How do i fit that sparql calculation? - syntax-error

Question

this is my actual problem:
?var0 is a group variable and ?var1 is not. But whenever I try to validate the syntax, there comes the following error message:
Non-group key variable in SELECT: ?var1 in expression ( sum(?var0) / ?var1 )
The complete Query:
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX cz: <http://www.vs.cs.hs-rm.de/ontostor/SVC#Cluster>
PREFIX n: <http://www.vs.cs.hs-rm.de/ontostor/SVC#Node>
SELECT ( (SUM(?var0) / ?var1) AS ?result)
WHERE{
?chain0 rdf:type rdfs:Property .
?chain0 rdfs:domain <http://www.vs.cs.hs-rm.de/ontostor/SVC#Cluster> .
?chain0 rdfs:range <http://www.vs.cs.hs-rm.de/ontostor/SVC#Node> .
?this ?chain0 ?arg0 .
?arg0 n:node_realtime_cpu ?var0 .
?this cz:node_count ?var1 .
}
My question is how to correct that calculation to fit the SPARQL syntax?

Answer 1

The immediate problem is that ?var1 is not grouped on, so a fix would be to simply append
GROUP BY ?var1
at the end of your query.
However, whether that gives you the calculation you actually want is another matter.
It's not quite clear what you're trying to calculate, but it looks as if you're attempting to determine the average node_realtime_cpu for a cluster. If that is the case, you can probably do your calculation by just using SPARQL's AVG function instead:
SELECT ( AVG(?var0) AS ?result)
WHERE{
?chain0 rdf:type rdfs:Property .
?chain0 rdfs:domain <http://www.vs.cs.hs-rm.de/ontostor/SVC#Cluster> .
?chain0 rdfs:range <http://www.vs.cs.hs-rm.de/ontostor/SVC#Node> .
?this ?chain0 ?arg0 .
?arg0 n:node_realtime_cpu ?var0 .
}
GROUP BY ?this // grouping on the cluster identifier so we get an average _per cluster_
Yet another alternative would be to keep your query as-is, but group on two variables:
GROUP BY ?this ?var1
Which is best depends on what your data looks like and what, exactly, you're trying to calculate.