Creating element declarations in XML Schema

One of the functions of a schema is to indicate which elements can be used in an XML document, where they should appear, and how many times they can be used. All of this information is included in an element declaration. There are two types of elements we can declare in an XML Schema: complex elements and simple elements. Let’s explore the two types of elements before adding element declarations to our XSD.

Simple elements

A simple element is an XML element that can contain only data - it can’t contain child elements or attributes. The following example shows one method for creating simple elements:

<xs:element name="animal" type="xs:string" />

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The first part of the declaration indicates that we’re declaring an element, and the second part indicates the name of the element. The third part of the declaration indicates the type of data that the element can contain —in this case, the element can contain text. There are many different data types, ranging from broad to specific, and the most common types are outlined in the following table:

Let’s practice declaring simple elements — we’ll start by declaring the child elements of <job>. To help us out as we add to the XSD file, we’ll add a note to indicate what type of declarations we’re adding.

Complex elements

<xs:element name="book">
	<xs:complexType>
		<xs:sequence>
			<xs:element name="title" type="xs:string"  />
			<xs:element name="author" type="xs:string" />
			<xs:element name="isbn" type="xs:integer" />
			<xs:element name="genre" type ="xs:string" maxOccurs="unbounded" />
		</xs:sequence>
	</xs:complexType>
</xs:element>

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The opening tag indicates the name of the element we’re declaring, and the following tag indicates that this element is a complex element. The next tag indicates that the child elements we include should be included in a specific order - the following table explains the different order indicators we can choose from:

Finally, the child elements are declared. There are two ways to indicate a complex element’s children: we can either declare the element inside the complex element, as shown in the previous example, or refer to a previously declared simple element, like in the following example:

	<xs:element ref="title" />

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Additionally, if using the sequence order indicator, we can indicate how often the elements can occur inside of the parent element using maxOccurs, which indicates the maximum number of times the element can appear, and minOccurs, which indicates the minimum number of times an element can appear. The occurrence indicator is included at the end of the element declaration, as seen in the following example:

	<xs:element name="genre" type="xs:string" maxOccurs="3" />

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NOTE: To indicate that an element can appear an unlimited number of times, the value for maxOccurs should be set to unbounded.

At this point, we’re ready to declare the complex elements we’ll be using. We'll be declaring <job> and making references to all the child elements it contains, which we declared previously as simple elements. We'll also be declaring the element <midwest-job-listing>, and indicating that its one child, <job>, can be repeated inside this element an unlimited amount of times.