$query = "
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX sioc:   <http://rdfs.org/sioc/ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

SELECT ?post ?title ?link ?date ?author
WHERE {
  ?post sioc:title ?title .
  ?post sioc:created_at ?date .
  ?post sioc:has_creator ?_y .
  ?_y rdfs:label ?author .
  OPTIONAL { ?_z sioc:has_reply ?post }
  FILTER (!bound(?_z))
}
ORDER BY ?date