im trying to use one of the filters not exists. has checked this query with sparql validator its syntax is right. im using Twinkle sparql tool.
# filename: ex067.rq
PREFIX ab: <http://learningsparql.com/ns/addressbook#>
SELECT ?first ?last
WHERE
{ ?s ab:firstName ?first .
?s ab:lastName ?last.
FILTER NOT EXISTS {?s ab:workTel ?workNum }
}