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精品文档 legacy database legacy database some other applications some other applications Interface user Stars(name, address(street, city)) Stars(name, street, city) root star star cf name address address mh name street city 精品文档 XML (Extensible Markup Language) XML is a tag-based notation designed originally for marking documents, much like HTML. While HTML’s tags talk about the presentation of the information contained in documents – for instance, which portion is to be displayed in italics or what the entries of a list are – XML tags intended to talk about the meanings of pieces of the document. Tags: opening tag - …. , e.g., Foo closing tag - / … , e.g., /Foo A pair of matching tags and everything that comes between them is called an element. 精品文档 XML with and without a schema XML is designed to be used in two somewhat different modes: Well-formed XML allows you to invent your own tags, much like the arc-labels in semistructured data. But there is no predefined schema. However, the nesting rule for tags must be obeyed, or the document is not well-formed. Valid XML involves a DTD (Document Type Definition) that specifies the allowed tags and gives a grammar for how they may be nested. This form of XML is intermediate between the strict-schema such as the relational model, and the completely schemaless world of semistructured data. 精品文档 ? Xml version = “1.0” encoding = “utf-8” standalone = “yes” ? StarMovieData Star NameCarrie Fishes/Name Address Street123 Maple St./StreetCityHollywood/City /Address Address Street5 Locust Ln./StreetCityMalibu/City Address /Star Star NameMark Hamill/NameStreet456 Oak Rd./Street CityBrentwood/City /Star Movie TitleStar Wars/titleYear1977/Year /Movie /StarMovieData 精品文档 Attributes As in HTML, an XML element can have attributes (name-value pairs) with its opening tag. An attribute is an alternative way to represent a leaf node of semistructured data. Attributes, like tags, can represent labeled arcs in
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