0.3 Graph Definitions An [URL=http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#dfn-rdf-graph]RDF graph[/URL], or simply a graph, is a set of RDF triples.
A subgraph of an RDF graph is a subset of the triples in the graph. A triple is identified with the singleton set containing it, so that each triple in a graph is considered to be a subgraph. A proper subgraph is a proper subset of the triples in the graph.
A ground RDF graph is one with no blank nodes.
A name is a URI reference or a literal. These are the expressions that need to be assigned a meaning by an [URL=http://www.w3.org/TR/rdf-mt/#glossInterpretation#glossInterpretation]interpretation[/URL]. Note that a typed literal comprises two [URL=http://www.w3.org/TR/rdf-mt/#defname#defname]name[/URL]s: itself and its internal type URI reference.
A set of [URL=http://www.w3.org/TR/rdf-mt/#defname#defname]name[/URL]s is referred to as a vocabulary. The vocabulary of a graph is the set of names which occur as the subject, predicate or object of any triple in the graph. Note that URI references which occur only inside typed literals are not required to be in the vocabulary of the graph.
[@图的基本定义: 一个三元组的集合; 名字(name)的定义: URIref和文字; 词汇集的定义: name的集合]
Suppose that M is a mapping from a set of blank nodes to some set of literals, blank nodes and URI references; then any graph obtained from a graph G by replacing some or all of the blank nodes N in G by M(N) is an instance of G. Note that any graph is an instance of itself, an instance of an instance of G is an instance of G, and if H is an instance of G then every triple in H is an instance of some triple in G.
[@图的实例: 把其中的空节点替换为name或空节点 ]
An instance with respect to a vocabulary V is an [URL=http://www.w3.org/TR/rdf-mt/#definst#definst]instance[/URL] in which all the [URL=http://www.w3.org/TR/rdf-mt/#defname#defname]name[/URL]s in the instance that were substituted for blank nodes in the original are [URL=http://www.w3.org/TR/rdf-mt/#defname#defname]name[/URL]s from V.
[@关于词汇集V的图实例: 用以替换的name来自V ]
A proper instance of a graph is an instance in which a blank node has been replaced by a name, or two blank nodes in the graph have been mapped into the same node in the instance.
[@图的真实例(proper instance): 至少有一个空节点别替换为名字,或两个空节点合并为一个]
Any instance of a graph in which a blank node is mapped to a new blank node not in the original graph is an instance of the original and also has it as an instance, and this process can be iterated so that any 1:1 mapping between blank nodes defines an instance of a graph which has the original graph as an instance. Two such graphs, each an instance of the other but neither a proper instance, which differ only in the identity of their blank nodes, are considered to be [URL=http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-graph-equality]equivalent[/URL]. We will treat such equivalent graphs as identical; this allows us to ignore some issues which arise from 're-naming' nodeIDs, and is in conformance with the [URL=http://www.w3.org/TR/rdf-mt/#nodeIDnote#nodeIDnote]convention[/URL] that blank nodes have no label. Equivalent graphs are mutual instances with an invertible instance mapping.
[@对空节点的1-1映射,如重命名nodeID, 仍然是等价的图]
An RDF graph is lean if it has no instance which is a proper subgraph of the graph. Non-lean graphs have internal redundancy and express the same content as their lean subgraphs. For example, the graph
<ex:a> <ex:p> _:x .
_:y <ex:p> _:x .
is not [URL=http://www.w3.org/TR/rdf-mt/#deflean#deflean]lean[/URL], but
[@ 因为<ex:a> <ex:p> _:x 是其真子图, 也是它的实例(把_:y 替换为<ex:a>) ]
<ex:a> <ex:p> _:x .
_:x <ex:p> _:x .
is [URL=http://www.w3.org/TR/rdf-mt/#deflean#deflean]lean[/URL].
[@图是紧致的(lean), 如果没有真子图(proper subgraph)也是它的实例(instance), 简单的判断方法: 枚举图的实例, 看是否同时也是图的真子图]
A merge of a set of RDF graphs is defined as follows. If the graphs in the set have no blank nodes in common, then the union of the graphs is a merge; if they do share blank nodes, then it is the union of a set of graphs that is obtained by replacing the graphs in the set by equivalent graphs that share no blank nodes. This is often described by saying that the blank nodes have been 'standardized apart'. It is easy to see that any two merges are equivalent, so we will refer to the merge, following the convention on equivalent graphs. Using the convention on equivalent graphs and identity, any graph in the original set is considered to be a subgraph of the merge.
[@图的合并: 1）如果没有共同的空节点（没有相同的空节点标识符）,是两个图对应的三元组集合的并集；2）如果有相同的空节点，通过1－1映射重新命名空节点，使得他们没有共同的空节点，然后取并集 ]
[@疑问： 在图语法中，并没有出现空节点标识符，因为无法判断什么是“blank nodes in common”？]
One does not, in general, obtain the merge of a set of graphs by concatenating their corresponding [URL=http://www.w3.org/TR/rdf-testcases/#ntriples]N-Triples[/URL] documents and constructing the graph described by the merged document. If some of the documents use the same node identifiers, the merged document will describe a graph in which some of the blank nodes have been 'accidentally' identified. To merge [URL=http://www.w3.org/TR/rdf-testcases/#ntriples]N-Triples[/URL] documents it is necessary to check if the same nodeID is used in two or more documents, and to replace it with a distinct nodeID in each of them, before merging the documents. Similar cautions apply to merging graphs described by RDF/XML documents which contain nodeIDs, see [URL=http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/]RDF/XML Syntax Specification (Revised)[/URL] [[URL=http://www.w3.org/TR/rdf-mt/#ref-rdf-syntax#ref-rdf-syntax]RDF-SYNTAX[/URL]].
[@对N－Triple的合并， 就要注意上面说的空节点标识符的问题， 因为空节点标识符都是局部的，只在其所在的图中有意义，合并的时候，如果不巧两个图有相同的空节点标识符，重新给一个就行了。]
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2004/8/3 23:01:00
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W3C规范《RDF Semantics》读书笔记
