PREFIX dc:     <http://purl.org/dc/elements/1.1/>
PREFIX stock:  <http://example.org/stock#>
PREFIX inv:    <http://example.org/inventory#>

SELECT ?book ?title
WHERE
{
  ?book dc:title ?title .
  ?book inv:price ?price .
  FILTER ?price < 15 .
  ?book inv:quantity ?num . 
  FILTER ?num > 0 .
}