Guide to using SPARQL inside XSLT

This guide demonstrates how Virtuoso allows SPARQL queries to be embedded into an XSLT style sheet, and executed against a remote SPARQL endpoint (here, we use the LOD Cloud Cache) to retrieve data for use in HTML output.

Implementation

• The key component of this feature is the <xsl:for-each-row> control element. This element has one attribute, named either "sparql" or "sql"; the value of the attribute is an XPath expression that returns the text of the query to executed, e.g.,
  

  
  <xsl:for-each-row sparql="string(\044query)">
  

  
  
• The XPath expression can be a plain string constant, but because the query can be quite long, it is usually more convenient to store the text in a variable and refer to that variable.
• The SPARQL query may contain parameters (written in the form ?:name). A parameter of this sort will get its value from the XSLT variable $name. (If there is any nesting or recursion, the value(s) will come from the innermost variables.)
• The query execution in turn creates new XSLT variables that exist inside the body of <xsl:for-each-row>. Each result column of a query becomes one XSLT variable.
• The body of <xsl:for-each-row> is instantiated once for each row of the query result set, with new values for the variables in each instantiation.

Example

1. Execute from iSQL:
   

   
   SQL>  xslt_sheet ('local://friends.xsl', 
                     xtree_doc ('<xsl:stylesheet xmlns:xsl = "http://www.w3.org/1999/XSL/Transform" version = "1.0" xmlns:sample="http://xslt/" >
                                 <xsl:output method = "html" version="1.0" doctype-public="http://public" doctype-system="http://system" />
                                 <xsl:param name="fname" select="''Kingsley''" />
                                 <xsl:param name="lname" select="''Idehen''" />
                                 <xsl:param name="fullname" select="''Kingsley Idehen''" />
                                 <xsl:variable name = "query">
   <![CDATA[
   PREFIX foaf: <http://xmlns.com/foaf/0.1/>
   SELECT DISTINCT ?maxlinkg 
                   ?fnick 
                   ?ffullname 
                   ?ffname 
                   ?flname 
                   ?fimg 
   WHERE
     {
       SERVICE <http://lod.openlinksw.com/sparql/> (DEFINE lang:dialect 65535)
         {
           { SELECT DISTINCT                                                     ?maxlinkg 
                                                                                 ?fg 
                                                                                 ?fnick
                                                                                 ?ffullname 
                             ?ffname (bif:__max_notnull (?flname1, ?flname2)) AS ?flname 
                                                                                 ?fimg
             WHERE
               {
                 { SELECT (MAX(?linkg)) AS ?maxlinkg 
                                           ?f 
                    WHERE
                      {
                        { SELECT DISTINCT ?k 
                           WHERE
                             { GRAPH ?kg 
                                 {
                                   { ?k  foaf:firstName   ?:fname  
                                      ;  foaf:familyName  ?:lname 
                                   }
                                   UNION
                                   { ?k  foaf:firstName    ?:fname
                                      ;  foaf:family_name  ?:lname 
                                   }
                                   UNION
                                   { ?k  foaf:name       ?:fullname } 
                             } 
                        } 
                      }
                    GRAPH ?linkg { ?k foaf:knows ?f } 
               } }
               GRAPH ?fg 
                 {               ?f  a                 <http://xmlns.com/foaf/0.1/Person>
                   .  OPTIONAL { ?f  foaf:firstName    ?ffname     }
                      OPTIONAL { ?f  foaf:nickname     ?fnick      }
                      OPTIONAL { ?f  foaf:name         ?ffullname  }
                      OPTIONAL { ?f  foaf:familyName   ?flname1    }
                      OPTIONAL { ?f  foaf:family_name  ?flname2    }
                      OPTIONAL { ?f  foaf:img          ?fimg       }
                 }
               FILTER ( bound ( ?ffullname ) 
                        || ( bound ( ?ffname ) 
                             && ( bound ( ?flname1 ) 
                                     || bound ( ?flname2 ) 
                              ) ) )
             } 
             ORDER BY ?fullname 
                      ?ffname 
           } 
         } 
     }
   ]]></xsl:variable>
   <xsl:template match = "/">
     <html><head>Simple demo for SPARQL inside XSLT</head>
       <body>
       <p>Known friends of <xsl:choose>
            <xsl:when test="\044fullname"><xsl:value-of select="\044fullname" /></xsl:when>
            <xsl:otherwise><xsl:value-of select="\044fname" /> <xsl:value-of select="\044lname" /></xsl:otherwise>
          </xsl:choose></p>
       <table>
       <xsl:for-each-row sparql="string(\044query)">
         <tr><td><xsl:if test="\044fimg"><a href="{\044fimg}" ><img src="{\044fimg}" width="64" height="64" /></a></xsl:if></td>
         <td><a href=".?fname={\044ffname}&lname={\044flname}&fullname={\044ffullname}">
         <b><xsl:choose>
            <xsl:when test="\044ffullname"><xsl:value-of select="\044ffullname" /></xsl:when>
            <xsl:otherwise><xsl:value-of select="\044ffname" /> <xsl:value-of select="\044flname" /></xsl:otherwise>
          </xsl:choose></b></a><br />
          <xsl:if test="\044fnick">Nick: <xsl:value-of select="\044fnick" /><br /></xsl:if>
          (data from <a href="{\044maxlinkg}" ><xsl:value-of select="\044maxlinkg" /></a> <!-- and <a href="{\044maxfg}" ><xsl:value-of select="\044maxfg" /></a>-->)
          </td></tr>
       </xsl:for-each-row>
       </table>
       </body>
     </html>
   </xsl:template>
   </xsl:stylesheet>'));
   
   DB.DBA.VHOST_DEFINE 
     ( lpath=>'/friends/', 
       ppath => '/!friends/', 
       is_dav => 1, 
       vsp_user => 'dba', 
       opts => vector('noinherit', 1)
     );
   
   CREATE PROCEDURE WS.WS."/!friends/" 
     ( INOUT  path    VARCHAR, 
       INOUT  params  ANY, 
       INOUT  lines   ANY
     )
   {
     http_value (xslt ('local://friends.xsl', xtree_doc ('<fake />'), params));
   }
   ;
   
   registry_set ('/!friends/', 'no_vsp_recompile')
   ;
   

   
   
2. Go to http://cname/friends/, where results should look something like this: