【71-75】
One of the difficulties in building an SQL-like query language for the Web is theabsence of a database （71）for this huge，heterogeneous repository of information.However，if we are interested in HTML documents only，we can construct a virtual schemafrom the implicit structure of these files. Thus,at the highest level of （72） ,every such document is identified by its Uniform Resource Locator （URL），has a （73）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 set of all HTML documents as a relation: Document（url, title,text,type，length，modif） Where all the （74） are characterstrings. In this framework,an individual document is identified with a （75） in thisrelation. Of course，if some optional information is missing from the HTML document,the associate fields will be left blank,but this is not uncommon in any database.
