2. 计算机
  3. One of the difficulties in building an SQL-like query lange for the Web is the absence of a database schema for this huge, heter

One of the difficulties in building an SQL-like query lange for the Web is the absence of a database schema for this huge, heter

admin2009-02-15  73
问题 One of the difficulties in building an SQL-like query lange for the Web is the absence of a database schema for this huge, heterogeneous repository of information. However, if we are interested in HTML documents only, we can construct a virtual(66)from the implicit structure of these files. Thus, at the highest level of(67), every such document is identified by its Uniform Resource Locator(URL), has a title and a text Also, Web servers provide some additional information such as the type, length, and the last modification date of a document. So, for data mining purposes, we can consider the site of all HTML documents as arelation:
   Document(url,(68), text, type, length, modify)
   Where all the(69)are character strings. In this framework, anindividual document is identified with a(70)in this relation. Of course, if some optional information is missing from the HTML document, the associate fields will de left blank, but this is not uncommon in any database.
选项 A、field
B、relation
C、script
D、tuple
答案D
解析 在万维网上建立一个类似于SQL的查询语言的困难之一是缺乏一种适用于这种巨大的、异构型信息仓库的数据库模式。然而，如果仅限于HTML文档，我们就可以由这种文件的隐含结构建立一种虚拟模式。这样，在最高抽象级别，每个文档都可以由统一资源定位器(URL)来标识，有一个标题和一个文本。同时，由Web服务器了来提供某些附加的信息，例如，类型、长度和文档的最后修改日期。这样，对于数据挖掘应用来说，我们可以把所有HTML文档的集合看做一个关系：
Document (ur1，title，text，type，length，modify)
   这里，所有的属性都是字符串。在这种框架下，一个单独的文档可以用这种关系的一个元组来标识。当然，如果某些任选信息在HTML文档中缺失，有关字段就留做空白，但这种情况在任何数据库中都是常见的。
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