【一日一段 －0717】
4.4 More on Structured Values: rdf:value
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL] noted that the RDF model intrinsically supports only binary relations; that is, a statement specifies a relation between two resources. For example, the statement:
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL]注意到RDF模型本质上只支持二元关系，也就是说，一个陈述表示了两个资源之间的关系。例如，陈述：
exstaff:85740 exterms:manager exstaff:62345 .
states that the relation exterms:manager holds between two employees (presumably one manages the other).
说两个雇员之间有exterms: manager关系（一个管理另一个）。
However, in some cases it is necessary to represent information involving higher arity relations (relations between more than two resources) in RDF. [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL] discussed one example of this, where the problem was to represent the relationship between John Smith and his address information, and the value of John's address was a structured value of his street, city, state, and postal code. Writing this as a relation shows that this address is a 5-ary relation of the form:
然而，有些情况下有必要在RDF中表示多元关系（多于两个资源之间的关系）。[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL] 讨论了一个例子，这个例子的问题是要表示John Smith和他的地址信息，即一个由街道，城市，州，邮政编码组成的结构。可以看出这个地址信息是一个5元关系。
address(exstaff:85740, "1501 Grant Avenue", "Bedford", "Massachusetts", "01730")
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL] noted that this kind of structured information can be represented in RDF by considering the aggregate thing be described (here, the group of components representing John's address) as a separate resource, and then making separate statements about that new resource, as in the triples:
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties]Section 2.3[/URL]注意到这种结构化的信息可以通过聚集一部分信息作为一个单独的资源（这里，John’s的地址信息），然后对这个新资源发表单独的陈述，如下面的三元组所示：
exstaff:85740 exterms:address _:johnaddress .
_:johnaddress exterms:street "1501 Grant Avenue" .
_:johnaddress exterms:city "Bedford" .
_:johnaddress exterms:state "Massachusetts" .
_:johnaddress exterms:postalCode "01730" .
(where _:johnaddress is the blank node identifier of the blank node representing John's address.)
This is a general way to represent any n-ary relation in RDF: select one of the participants (John in this case) to serve as the subject of the original relation (address in this case), then specify an intermediate resource to represent the rest of the relation (either with or without assigning it a URI), then give that new resource properties representing the remaining components of the relation.
（其中_:johnaddress是表示John’s的地址的匿名节点的标识）这是在RDF中表示N元关系的一般方法：选出关系中的一个元素（这里是John）作为关系（这里是address）的主体，指定一个中间的资源来表示关系中剩下的部分，然后把关系剩下部分都作为这个中间资源的属性。
In the case of John's address, none of the individual parts of the structured value could be considered the "main" value of the exterms:address property; all of the parts contribute equally to the value. However, in some cases one of the parts of the structured value is often thought of as the "main" value, with the other parts of the relation providing additional contextual or other information that qualifies the main value. For instance, in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example9]Example 9[/URL] in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#newresources]Section 3.2[/URL], the weight of a particular tent was given as the decimal value 2.4 using a typed literal, i.e.,
在John 的地址信息中，没有一个部分能够作为属性exterms:address的主值（main value）,所有部分对值的贡献是相同的。然后，有时候，结构中的一部分可以作为结构的主值，其他部分提供上下文信息或其他对主值的修饰信息。例如，在[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#newresources]Section 3.2[/URL]的[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example9]Example 9[/URL] 中，一个帐篷的重量的值是小数2.4,也就是，
exproduct:item10245 exterms:weight "2.4"^^xsd:decimal .
In fact, a more complete description of the weight would have been 2.4 kilograms rather than just the decimal value 2.4. To state this, the value of the exterms:weight property would need to have two components, the typed literal for the decimal value and an indication of the unit of measure (kilograms). In this situation the decimal value could be considered the "main" value of the exterms:weight property, because frequently the value would be recorded simply as the typed literal (as in the triple above), relying on an understanding of the context to fill in the unstated units information.
实际上，更为复杂的对重量的描述应该是2.4千克（ kilograms），而不是仅仅小数2.4。为了表示这个，属性exterms:weight的值还需要2个部分：作为类型文字的数值和量度的单位（千克）。在这种情况下，数值可以看成属性exterms:weight的主值，因为一般这个值都是一个类型文字，而依赖山下文信息来填充这个值的单位。
In the RDF model a qualified property value of this kind can be considered as simply another kind of structured value. To represent this, a separate resource could be used to represent the structured value as a whole (the weight, in this case), and to serve as the object of the original statement. That resource could then be given properties representing the individual parts of the structured value. In this case, there should be a property for the typed literal representing the decimal value, and a property for the unit. RDF provides a predefined rdf:value property to describe the main value (if there is one) of a structured value. So in this case, the typed literal could be given as the value of the rdf:value property, and the resource exunits:kilograms as the value of an exterms:units property (assuming the resource exunits:kilograms is defined as part of example.org's vocabulary). The resulting triples would be:
在RDF模型中，这种类型的带有修饰的属性值可以看成一种简单的结构化值。为了表达这个，一个单独的资源可以用来表示结构化值，且作为原来陈述的客体。这个资源可以有属性表示结构值中的部分。在这里，应该有个属性表示数值，还一个属性表示单位。RDF提供了一个预定义的属性rdf:value来描述结构值中的主值。因此，这里的数值可以作为rdf:value的值，资源exunits:kilograms作为属性exterms:units的值（假设资源exunits:kilograms定义为example.org的词汇集的一部分）。结果，这些三元组为：
exproduct:item10245 exterms:weight _:weight10245 .
_:weight10245 rdf:value "2.4"^^xsd:decimal .
_:weight10245 exterms:units exunits:kilograms .
which can be expressed using the RDF/XML shown in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL]:
这可以用RDF/XML表示，如[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL]所示：
Example 21: RDF/XML using rdf:value
<?xml version="1.0"?>
<!DOCTYPE rdf:RDF [<!ENTITY xsd "[URL=http://www.w3.org/2001/XMLSchema#]http://www.w3.org/2001/XMLSchema#[/URL]">]>
<rdf:RDF xmlns:rdf="[URL=http://www.w3.org/1999/02/22-rdf-syntax-ns#]http://www.w3.org/1999/02/22-rdf-syntax-ns#[/URL]"
xmlns:exterms="[URL=http://www.example.org/terms/]http://www.example.org/terms/[/URL]">
<rdf:Description rdf:about="[URL=http://www.example.com/2002/04/products#item10245]http://www.example.com/2002/04/products#item10245[/URL]">
<exterms:weight rdf:parseType="Resource">
<rdf:value rdf:datatype="&xsd;decimal">2.4</rdf:value>
<exterms:units rdf:resource="[URL=http://www.example.org/units/kilograms]http://www.example.org/units/kilograms[/URL]"/>
</exterms:weight>
</rdf:Description>
</rdf:RDF>
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL] also illustrates a second use of the rdf:parseType attribute introduced in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#collections]Section 4.2[/URL], in this case, rdf:parseType="Resource". An rdf:parseType="Resource" attribute is used to indicate that the contents of an element are to be interpreted as the description of a new (blank node) resource, without actually having to write a nested rdf:Description element. In this case, the rdf:parseType="Resource" attribute used in the exterms:weight property element indicates that a blank node is to be created as the value of the exterms:weight property, and that the enclosed elements (rdf:value and exterms:units) describe properties of that blank node. Further details on rdf:parseType="Resource" are given in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#ref-rdf-syntax][RDF-SYNTAX[/URL]].
[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL]也显示了对[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#collections]Section 4.2[/URL]介绍的属性rdf:parseType的用法，在这里，rdf:parseType="Resource"用以表示节点的内容同时被解释为一个新的匿名节点，而不用写一个嵌套的rdf:Description元素。rdf:parseType="Resource" 用在属性 exterms:weight 元素节点中表示创建了一个匿名节点作为属性的值，且包含的元素 (rdf:value 和 exterms:units) 描述了这个匿名节点的属性。关于rdf:parseType="Resource"更详细的信息在[RDF-SYNTAX].
The same approach can be used to represent quantities using any units of measure, as well as values taken from different classification schemes or rating systems, by using the rdf:value property to give the main value, and using additional properties to identify the classification scheme or other information that further describes the value.
相同的方法也可用于表示具有任何单位的数量，用rdf:value属性表示主值，用其他的属性表示关于数量的其他信息。
There is no need to use rdf:value for these purposes (e.g., a user-defined property name, such as exterms:amount, could have been used instead of rdf:value in [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL]), and RDF does not associate any special meaning with rdf:value. rdf:value is simply provided as a convenience for use in these commonly-occurring situations.
但没有必要用rdf:value去取代用户自定义的属性，如[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#example21]Example 21[/URL]中的属性exterms:amount, 因为RDF并没有给rdf:value特殊的解释。它只是作为表示这种情形的惯例。
However, even though much existing data in databases and on the Web (and in later Primer examples) takes the form of simple values for properties such as weights, costs, etc., the principle that such simple values are often insufficient to adequately describe these values is an important one. In a global environment such as the Web, it is generally not safe to make the assumption that anyone accessing a property value will understand the units being used (or other contextually-dependent information that may be involved). For example, a U.S. site might give a weight value in pounds, but someone accessing that data from outside the U.S. might assume that weights are given in kilograms. The correct interpretation of data in the Web environment may require that additional information (such as units information) be explicitly recorded. This can be done in many ways, such as using rdf:value, building units into property names (e.g., exterms:weightInKg), defining specialized datatypes that include units information (e.g., extypes:kilograms), or adding additional user-defined properties to specify this information (e.g., exterms:unitOfWeight), either in descriptions of individual items or products, in descriptions of sets of data (e.g., all the data in a catalog or on a site), or in schemas (see [URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#rdfschema]Section 5[/URL]).
然而，尽管大部分数据库中和Web上的数据用简单的数值作为属性（如，重量，价格等）的值，这种简单的数值往往不足够描述这些值。在像Web这样的环境，一般来说，假设其他人访问属性的值的时候能知道它的单位（或其他上下文相关的信息）是不稳妥的。例如，一个美国的网站给中重量的单位是磅，但不是美国的人访问这个数据的时候常假设单位是公斤。对Web上数据的正确解释可能需要其他的信息，如单位信息。这有很多种方法：例如，用rdf:value,把单位信息放在属性名字种，如属性exterms:weightInKg包含了数据的单位信息。或者，加上用户自定义的的信息，如exterms:unitOfWeight, 在物品或产品实例的描述中，在一个数据集的描述中，或者在Schema中（参考[URL=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#rdfschema]Section 5[/URL]）
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