#title: Notable books on linked data, semantic technologies, and semantic knowledge graphs 
 
SELECT DISTINCT ?book ?title (GROUP_CONCAT (DISTINCT ?author; SEPARATOR = ", ") AS ?authors) ?pubDate
{
  VALUES ?book {wd:Q17744504 wd:Q17744476 wd:Q61718117 wd:Q59784970 wd:Q17744339 wd:Q51601248 wd:Q17744488 wd:Q27942623  wd:Q108378522 wd:Q110549106}
         
         ?book wdt:P1476 ?title.
         OPTIONAL {?book (wdt:P50/rdfs:label)|(wdt:P98/rdfs:label)|wdt:P2093 ?author
                   FILTER (LANG(?author)=""||LANG(?author)="en")}
         OPTIONAL {?book wdt:P577 ?pubDate}

}
GROUP BY ?book ?title ?pubDate

#note: this is a snippet, not a valid query

VALUES (?x ?y ?z) {(:URI1 "ex" 1) (:URI2 "wye" 2) (:URI3 "zed" UNDEF)}

• The third list has no value for ?z, shown by the keyword UNDEF