Detailed explanation of the basic knowledge of using XML Schema to define elements (pictures and text)

The new XML Schema system is about to become a W3C recommended standard, with the purpose of overcoming the limitations of DTD (see sidebar, Limitations of DTD) and providing rich grammatical structures for XML documents. This article demonstrates the flexibility of schemas and explains how to use the XML Schema system to define the most basic building blocks of XML documents - elements.

XML Schema is more powerful than DTD. To illustrate the power of the XML Schema mechanism, the following three program listings briefly compare different ways of representing elements. Listing 1 gives an XML document segment, Listing 2 declares these two elements using DTD syntax, and Listing 3 is the corresponding XML Schema syntax form. Note that the same XML syntax is used in Listing 3. Through the schema, the validation parser can check whether the element InvoiceNo is a positive integer and whether the first character of the ProductID element is a letter from A to Z, followed by six Arabic digits. In contrast, a validating parser that references a DTD can only check whether the elements are represented as strings.

Listing 1: XML document segment

123456789
J123456

Listing 2: DTD segment describing the elements in Listing 1

Listing 3: XML Schema describing the elements in Listing 1

Using namespaces in XML Schema

In this collaborative world, one person may be working with documents from multiple other groups, and different groups may want to represent their documents in different ways. data elements. Additionally, they may reference elements in a document with the same name created by different groups. How to distinguish different definitions of the same name? XML Schema uses namespaces to distinguish these definitions.

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Limitations of DTD

(Although DTD has successfully served SGML and HTML developers for 20 years as a mechanism for describing structured information, But compared with XML Schema, it has serious limitations.

DTD requires elements to be composed of the following three components:

Text string

Text string and A mix of other child elements

A set of child elements

The DTD does not use XML syntax and provides only limited support for types and namespaces)

a given XML. Schema defines a set of new names, such as element names, type names, attribute names, and attribute group names. The definitions and declarations of these names are written in the schema. Listing 3 defines names including InvoiceNo, ProductID, and ProductCode.

We say that a name defined in a schema belongs to its target namespace. The namespace itself has a fixed but unrestricted name that must conform to URL syntax. For example, for the schema section in Listing 3, you could name the namespace: http://www.SampleStore.com/Account.

The namespace name syntax is confusing because, although it starts with http://, that URL does not point to a file containing the schema definition. In fact, the URL http://www.SampleStore.com/Account does not point to any file at all, just an assigned name.

Definitions and declarations in the schema may reference names belonging to other namespaces. In this article, we call these namespaces source namespaces. Each schema has a target namespace, but there may be multiple source namespaces. Namespace names can be quite long, but abbreviations are available in XML documents through xmlns declarations. To illustrate these concepts, we can add a little more to the example pattern in Listing 4 mentioned above.

Listing 4: Target namespace and source namespace

In the XML Schema of Listing 4, the name of targetNamespace is www.SampleStore.com/Account, which contains the names InvoiceNo, ProductID and ProductCode. The names schema , element , simpleType , pattern , string , and positive-integer belong to the source namespace www.w3.org/1999/XMLSchema , abbreviated to xsd via the xmlns declaration. There is nothing special about the alias xsd, we can choose any other name. For convenience and simplicity later in this article, we use xsd to represent the namespace www.w3.org/1999/XMLSchema and omit the qualifier xsd in some code snippets. In this example, targetNamespace occasionally serves as a source namespace because other names are defined using the name ProductCode.

The schema section in Listing 4 does not need to specify the location of the source schema file. For the whole "schema of schemas", http://www.w3.org/1999/XMLSchema, there is no need to specify the location because its location is well known. For the source namespace www.SampleStore.com/Account , there is also no need to specify the location since it happens to be the target namespace defined in the file. To better understand how to specify the location of a schema and use the default namespace, take a look at the extended example in Listing 5.

Listing 5: Multiple source namespaces, importing one namespace

清单 5中多了一个名称空间引用： www.PartnerStore.com/PartsCatalog 。这个名称空间不同于 targetNamespace 和标准名称空间。因此必须使用 import 声明元素引入，该元素的 schemaLocation 属性指明包含模式的文件位置。默认的名称空间是www.w3.org/1999/XMLSchema ，它的 xmlns 声明没有名字。每个非限定的名字如 schema 和 element ，都属于默认名称空间www.w3.org/1999/XMLSchema 。如果模式从一个名称空间中引用了多个名字，将其指定为默认名字空间更方便。

一个 XML 实例文档可能引用多个名称空间的元素名，这些名称空间定义在不同模式中。为了引用和简化名称空间的名字，同样要使用 xmlns 声明。我们使用 XML Schema 实例名称空间的 schemaLocation 属性指定文件的位置。要注意，该属性不同于上一个例子中 xsd 名称空间的同名属性 schemaLocation 。

清单 6：使用来自多个模式的多个名称空间的名字

123456789

图 2：清单 5 和清单 6 的名称空间

定义元素

定义元素就是定义元素的名字和内容模型。在 XML Schema 中，元素的内容模型由其类型定义，因此 XML 文档中实例元素的值必须符合模式中定义的类型。

类型包括简单类型和复杂类型。简单类型的值不能包含元素或属性。复杂类型可以产生在其他元素中嵌套元素的效果，或者为元素增加属性。（到目前为止本文中的例子都是用户定义的简单类型，比如 ProductCode ）。XML Schema 规范也包括预定义的简单类型（请参阅侧栏 简单类型）。 派生的简单类型约束了基类型的值。比如，派生简单类型 ProductCode 的值是基类型 string 值的子集。

简单的、非嵌套的元素是简单类型

不含属性或其他元素的元素可以定义为简单类型，无论是预定义的简单类型还是用户定义的简单类型，如 string 、 integer 、 decimal 、 time 、 ProductCode 等等。

清单 7：一些元素的简单类型

带有属性的元素必须是复杂类型

现在，试着向 清单 7中的简单元素 price 增加属性 currency 。您不能这样做，因为简单类型的元素不能有属性。如果希望增加属性，您必须把 price 元素定义成复杂类型。在 清单 8的例子中，我们定义了一个 匿名类型，没有明确地命名这个复杂类型。换句话说，没有定义复杂类型 complexType 的 name 属性。

清单 8：一个复杂元素类型

嵌入其他元素的元素必须是复杂类型

在 XML 文档中，一个元素可能嵌入其他的元素。这种要求可以在 DTD 中直接表示。但 XML Schema 定义一个元素，这个元素有一个类型，而这个类型可以包含其他元素和属性的声明。 表 1给出了一个简单的例子。

表 1：DTD 和 XML Schema 中复杂数据类型的比较

XML 文档