Welcome to our free Learn XML tutorial. This tutorial is based on Webucator's Introduction to XML Training course.
In this lesson you will learn about XML Schemas and how they compare to DTDs.
XML Schema is an XML-based language used to create other XML-based languages and data models. An XML schema defines element and attribute names for a class of XML documents. The schema also specifies the structure that those documents must adhere to and the type of content that each element can hold.
You might be thinking, isn't this what DTDs are? That's right! But XML Schemas are more powerful than DTDs (more on that later in the lesson).
XML documents that attempt to adhere to an XML schema are said to be instances of that schema. If they correctly adhere to the schema, then they are valid instances. This is not the same as being well formed. A well-formed XML document follows all the syntax rules of XML, but it does not necessarily adhere to any particular schema. So, again, an XML document can be well formed without being valid, but it cannot be valid unless it is well formed.
DTDs are similar to XML schemas in that they are used to create classes of XML documents. DTDs were around long before the advent of XML. They were originally created to define languages based on SGML, the parent of XML. Although DTDs are still common, XML Schema is a much more powerful language.
As a means of understanding the power of XML Schema, let's look at the limitations of DTD.
An XML schema describes the structure of an XML instance document by defining what each element must or may contain. An element is limited by its type. For example, an element of complex type can contain child elements and attributes, whereas a simple-type element can only contain text. The diagram below gives a first look at the types of XML Schema elements.
Note: we will review this in the next presentation.
Schema authors can define their own types or use the built-in types. Throughout this course, we will refer back to this diagram as we learn to define elements. You may want to save this diagram (right-click the image and select "Save Image As..."), so that you can easily reference it.
The following is a high-level overview of schema types.
Let's take a look at a simple XML schema, which is made up of one complex-type element with two child simple-type elements.
<?xml version="1.0" encoding="UTF-8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="Author"> <xs:complexType> <xs:sequence> <xs:element name="FirstName" type="xs:string" /> <xs:element name="LastName" type="xs:string" /> </xs:sequence> </xs:complexType> </xs:element> </xs:schema>
As you can see, an XML schema is an XML document and must follow all the syntax rules of any other XML document; that is, it must be well formed. XML schemas also have to follow the rules defined in the "Schema of schemas," which defines, among other things, the structure of an element and attribute names in an XML schema.
Although it is not required, it is a common practice to use the
xsqualifier to identify schema elements and types.
The document element of XML schemas is
xs:schema. It takes the attribute
xmlns:xs with the value of
http://www.w3.org/2001/XMLSchema, indicating that the document should follow the rules of XML Schema. This will be clearer after you learn about namespaces.
In this XML schema, we see a
xs:element element within the
xs:element is used to define an element. In this case it defines the element
Author as a complex-type element, which contains a sequence of two elements:
LastName, both of which are of the simple type, string.
In the last section, you saw an example of a simple XML schema, which defined the structure of an
Author element. The code sample below shows a valid XML instance of this XML schema.
<?xml version="1.0"?> <Author xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="Author.xsd"> <FirstName>Mark</FirstName> <LastName>Twain</LastName> </Author>
This is a simple XML document. Its document element is Author, which contains two child elements:
LastName, just as the associated XML schema requires.
xmlns:xsi attribute of the document element indicates that this XML document is an instance of an XML schema. The document is tied to a specific XML schema with the
There are many ways to validate the XML instance. If you are using an XML authoring tool, it very likely is able to perform the validation for you. Alternatively, there is a simple online XML Schema validator tool listed below.
In this exercise, you will write an XML Schema for the business letter shown below. You will then give your schema to another student, who will mark up the business letter as a valid XML document according to your schema. Likewise, you will markup the business letter according to someone else's schema. Make sure that the XML file contains the xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" and xsi:noNamespaceSchemaLocation="your-schema-file-name.xsd" attributes in the document element.
Both documents should be saved in the SchemaBasics/Exercises folder. To test whether the XML file is valid, you can use the XMLSpy editor.
November 29, 2011 Joshua Lockwood Lockwood & Lockwood 291 Broadway Ave. New York, NY 10007 United States Dear Mr. Lockwood: Along with this letter, I have enclosed the following items: - two original, execution copies of the Webucator Master Services Agreement - two original, execution copies of the Webucator Premier Support for Developers Services Description between Lockwood & Lockwood and Webucator, Inc. Please sign and return all four original, execution copies to me at your earliest convenience. Upon receipt of the executed copies, we will immediately return a fully executed, original copy of both agreements to you. Please send all four original, execution copies to my attention as follows: Webucator, Inc. 4933 Jamesville Rd. Jamesville, NY 13078 USA Attn: Bill Smith If you have any questions, feel free to call me at 800-555-1000 x123 or e-mail me at email@example.com. Best regards, Bill Smith VP, Operations