I would like to ask about the semantics that the GROUP_CONCAT function comes with.
I experimented a little bit with Jena and if I use UNIONs and GROUP_CONCAT then I am getting an error. I know that UNION would also give for multiple variables a lot of null values. I thought that GROUP_CONCAT would have to problem to take them and do something like: (null, null, dbpedia:Peter). Can't UNION and G_C exist and work together? I always have to use joins (.) to connect triple patterns?
An alternative of course whould be to join the triple patterns. However, the problem using joins of triple patterns is that if a variable returns nothing as result, even if the variables are not practically joined with each other since they have nothing in common, then the whole query returns a null value as a result.
P.S.: One would suggest to use OPTIONAL. I already asked a question about OPTIONAL and I don't want to use it for some reasons. I want to use a set of UNIONs however together with GROUP_CONCAT since this function is very helpful for me.
and look at the rows.
Now when the whole thing gets aggregated (you have an aggregation in the
Try pushing the GROUP_CONCAT into the UNION:
answered 10 Jan '13, 16:16