Select inside another select

Question

I have the following query where I'm trying to use the id of an element from one graph to retrieve some values in another graph. However this doesn't work:

select * from <mygraph2#> where {
  ?s ?p ?id in {select ?id from <mygraph1#> where { ?id ?t "MyService" }
} 

Answer 1

You don't select from a subselect. You just execute the sub-select, and it provides some variables to the enclosing query. I'm not sure where you found an example like what you showed in the question. It's not what any of the examples in the standard (see Section 12, Subqueries) look like. Your query would be something like this (note that this isn't actually legal, since you can't use FROM in subqueries):

select * from <mygraph2#> where {
  ?s ?p ?id
  { select ?id from <mygraph1#>
    where { ?id ?t "MyService" } }
} 

However, the the graphs that you're selecting from are available as named graphs in the dataset, there's no real need for the sub-select here. You can just do

select * where {
  graph <mygraph2#> { ?s ?p ?id }
  graph <mygraph1#> { ?id ?t "MyService" }
}