---------------------------------
而应该得到的结果确是
---------------------------------
| resource                      |
=================================
| <http://somewhere/JohnSmith/> |
---------------------------------
此外，还有一个类似的问题，在5.1节中，
PREFIX info:        <http://somewhere/peopleInfo#>
PREFIX vcard:      <http://www.w3.org/2001/vcard-rdf/3.0#>
SELECT ?name ?age
WHERE {    ?person vcard:FN  ?name .
OPTIONAL { ?person info:age ?age . FILTER ( ?age > 24 ) }}
这个的执行结果我的是
-----------------------
| name          | age |
=======================
| "Becky Smith" |     |
| "Sarah Jones" |     |
| "John Smith"  |     |
| "Matt Jones"  |     |
-----------------------
而应该得到的是
-----------------------
| name          | age |
=======================
| "Becky Smith" |     |
| "Sarah Jones" |     |
| "John Smith"  | 25  |
| "Matt Jones"  |     |
-----------------------
从表面上看我认为应该是filter出了错，但是怎么改都不对，希望站长看到指点一下，谢谢！
还有，6.4中的
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#>
SELECT ?name1 ?name2
WHERE{  ?x a foaf:Person  
OPTIONAL { ?x  foaf:name  ?name1 }
  OPTIONAL { ?x  vCard:FN   ?name2 }}
我的执行结果是
---------------------------------
| name1         | name2|
=================================
---------------------------------
应该得到的确是
---------------------------------
| name1         | name2|
=================================
| "Matt Jones"  |               |
| "Sarah Jones" |               |
|               | "Becky Smith" |
|               | "John Smith"  |
---------------------------------
请指点！