%META:TOPICPARENT{name="VirtTipsAndTricksGuide"}%

---++What is the difference between the functions SAMPLE, <nowiki>GROUP_CONCAT</nowiki> and <nowiki>GROUP_DIGEST</nowiki>?

This example demonstrates the differences between the functions <code>[[http://docs.openlinksw.com/virtuoso/fn_SAMPLE.html][SAMPLE]]</code>, <code>[[http://docs.openlinksw.com/virtuoso/fn_GROUP_CONCAT.html][<nowiki>GROUP_CONCAT</nowiki>]]</code> and <code>[[http://docs.openlinksw.com/virtuoso/fn_GROUP_DIGEST.html][<nowiki>GROUP_DIGEST</nowiki>]]</code>.

Assume the following query that should get all <code>?assets</code> as a list with delimiters:

<verbatim>
SPARQL SELECT ?view ?path (sql:GROUP_CONCAT (?asset, ' ')) as ?asset_list
FROM <http://mygraph.com> 
WHERE
 { 
   ?view <viewPath> ?path ; 
     <viewContent> ?asset ; 
     <viewType>  'phyview'. 
 }
;
</verbatim>


This method is not universal, because conversion to strings will eliminate the difference between strings and IRIs and there should be some delimiter that never appears in values of <code>?asset</code>. In addition, the query may fail with "row too long" error if values of <code>?asset</code> are lengthy and/or numerous enough. It is also possible the query not work completely free from duplicates if more than one list is desired. E.g.:

<verbatim>
SPARQL 
SELECT ?view (sql:GROUP_CONCAT (?path, ' ')) as ?path_list
  (sql:GROUP_CONCAT (?asset, ' ')) as ?asset_list
FROM <http://mygraph.com>
WHERE 
  { 
    ?view <viewPath> ?path ; 
      <viewContent> ?asset ; 
      <viewType> 'phyview' . 
  }
</verbatim>

will not contain duplicates in lists only if either <code>?path</code> or <code>?asset</code> is unique for every found <code>?view</code>; but if it's so unique then there's no need in the corresponding <code>sql:[[http://docs.openlinksw.com/virtuoso/fn_GROUP_CONCAT.html][<nowiki>GROUP_CONCAT</nowiki>]]()</code> .

If there are many values per property but it's enough to return any single value and ignore the rest then use <code>sql:[[http://docs.openlinksw.com/virtuoso/fn_SAMPLE.html][SAMPLE]]()</code> function instead of <code>sql:[[http://docs.openlinksw.com/virtuoso/fn_GROUP_CONCAT.html][<nowiki>GROUP_CONCAT</nowiki>]]() </code>.

If there are many values per property and it's better to show more than one value but "row too long" error happens, then the <code>sql:[[http://docs.openlinksw.com/virtuoso/fn_GROUP_DIGEST.html][<nowiki>GROUP_DIGEST</nowiki>]]</code> function can be used:

<verbatim>
SPARQL 
SELECT ?view (sql:GROUP_DIGEST (?path, ' ', 1000, 1)) as ?path_list 
  (sql:GROUP_DIGEST (?asset, ' ', 1000, 1)) as ?asset_list
FROM <http://mygraph.com> 
WHERE
  { 
    ?view <viewPath> ?path ; 
      <viewContent> ?asset ; 
      <viewType> 'phyview' . 
  }
</verbatim>


---++Related

   * [[VirtTipsAndTricksGuide][Virtuoso Tips and Tricks Collection]]
   * [[http://docs.openlinksw.com/virtuoso/fn_SAMPLE.html][SAMPLE]]
   * [[http://docs.openlinksw.com/virtuoso/fn_GROUP_CONCAT.html][<nowiki>GROUP_CONCAT</nowiki>]]
   * [[http://docs.openlinksw.com/virtuoso/fn_GROUP_DIGEST.html][<nowiki>GROUP_DIGEST</nowiki>]]