sparql query with sub-query is not working as expected

[wrobell]

Please see the example in https://www.w3.org/TR/sparql11-query/#subqueries.

The result of the script below should resemble

y           minName
:bob     "B. Bar"
:carol   "C. Baz"

But we get null instead of minName values

[
    (rdflib.term.URIRef('http://people.example/bob'), None),
    (rdflib.term.URIRef('http://people.example/carol'), None)
]

The script

import rdflib
import io

data = """
@prefix : <http://people.example/> .

:alice :name "Alice", "Alice Foo", "A. Foo" .
:alice :knows :bob, :carol .
:bob :name "Bob", "Bob Bar", "B. Bar" .
:carol :name "Carol", "Carol Baz", "C. Baz" .
"""

graph = rdflib.Graph()

f = io.StringIO(data)
graph.parse(f, format='n3')

result = graph.query("""
PREFIX : <http://people.example/>
PREFIX : <http://people.example/>

SELECT ?y ?minName
WHERE {
  :alice :knows ?y .
  {
    SELECT ?y (MIN(?name) AS ?minName)
    WHERE {
      ?y :name ?name .
    } GROUP BY ?y
  }
}
""")

print(list(result))

[gromgull]

related #607?