jena2.5.4推理机系统的一种最基本实现 HP实验室出品

				# An optional and an fixed pattern
				PREFIX : 
				
				# Ensure an optional does not mask a later one.
				
				# This query works unreordered because it has written out the needed UNION
				
				SELECT * 
				WHERE 
				{
				  :x :p ?v .
				  OPTIONAL {:y ?p ?a } 
				  OPTIONAL {:z ?p ?v }
				}
				
				## SELECT DISTINCT *
				## {
				##   :x :p ?v .
				##     { OPTIONAL {:y ?p ?a }  OPTIONAL {:z ?p ?v } }
				##   UNION
				##     { OPTIONAL {:z ?p ?v }  OPTIONAL {:y ?p ?a } }
				## }