Let's have a look at this XML documents called "shiporder.xml":

The XML document above consist of a root element, "shiporder", that contains a required attribute called "orderid". The "shiporder" element contain three different child elements: "orderperson", "shipto" and "item". The "item" element appear twice, and it contains a "title", an optional "note" element, a "quantity", and a "price" element.

The line above: xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance " tells the XML parser that this documents should be validated against a schema. The line: xsi:noNamespaceSchemaLocation="shiporder.xsd" specify WHERE the schema resides (here it is in the same folder as "shiporder.xml").

Now we want to create a schema for the XML document above.

We start by opening a new file that we will call as "shiporder.xsd". To create the schema we could simply follow the structure in the XML documents and define each element as we find it. We will start with the standard XML declaration followed by the xs:schema element that define a schema:

In the schema above we use the standard namespace (xs), and the URI associated with this namespace is the Schema language definition, which has the standard value of http://www.w3.org/2001/XMLSchemas.

Next, we have to define "shiporder" element. This element has an attribute and it contain other elements, therefore we consider it as a complex type. The child element of the "shiporder" element is surrounded by a xs:sequence element that defines an ordered sequence of sub elements:

Then we have to define the "orderperson" element as simple type (because it does not contain any attributes or other elements). The type (xs:string) is prefixed with the namespace prefix associated with XML Schemas that indicates a predefined schema data type:

Next, we have to defines two elements that are of the complex type: "shipto" and "item". We start by defining the "shipto" element as follows:

With schema we can define the number of possible occurrences for an element with the maxOccurs and minOccurs attributes. maxOccurs specify the maximum number of occurrences for an element and minOccurs specifies the minimum number of occurrences for an element. The default value for both maxOccurs and minOccurs is always 1!

Now we can define "item" element. This element can appear multiple times inside the "shiporder" element. This is specified by setting the maxOccurs attribute of the "item" element to "unbounded" which mean that there can be as many occurrences of the "item" element as the author wishes. Notice that the "note" element is optional here. We have specified this by setting the minOccurs attribute to be zero:

We can now declare the attribute of "shiporder" element. Since this is a required attribute we specify use="required".

Note: The attribute declaration must always come last:

Here is the complete listing of the schema file called the "shiporder.xsd":

The previous design method is very simple, but can be difficult to read and maintain when document are complex.

The next design method is based on defining all elements and attributes first, and then referring to them using the ref attributes.

Here is the new design of schema file ("shiporder.xsd"):

The third design method define classes or types, that enables us to reuse element definitions. This is done by naming the simpleTypes and complexTypes element, and then point to them through the type attribute of the element.

Here is the third design of the schemas file ("shiporder.xsd"):

The restriction element indicate that the datatype is derived from a W3C XML Schema namespace datatype. So, the following fragment mean that the value of the element or attribute must be a string value:

The restriction element is more often used to apply restriction to elements. Look at the following line from the schema above:

This indicate that the value of the element or attribute must be a string, it must be exactly six characters in a row, and those characters must be a number from 0 to 9.