PREFIX  dc: <http://purl.org/dc/elements/1.1/>
PREFIX  x: <http://example.org/ns#>
SELECT  ?title ?price
WHERE
    { ?book dc:title ?title . 
      OPTIONAL
        { ?book x:price ?price . 
          FILTER (?price < 15 && ?title = "TITLE 2") .
        } .
    }
