Introduction to the W3C XML Schema Recommendation

Introduction to the W3C XML Schema Recommendation

In May of 2001, the W3C finalized its recommendation for the XML Schema Definition Language. This standard allows an author to define simple and complex elements and the rules governing how those elements and their attributes may show up within an instance document. The author has a large amount of control over how the structure of a conform-ing XML document must be created. The author can apply various restrictions to the ele-ments and attributes within the document, from specifying the length to specifying an enumerated set of acceptable values for the element or attribute. With the XML Schema Definition Language, an XML schema author possesses an incredible amount of control over the conformance of an associated XML document to the specified schema. You can find more information on the W3C at www.w3c.org. Additionally, the W3C XML Schema recommendation is separated into three main documents:

•     The XML Schema Primer (Part 0), which can be found at http://www.w3c.org/TR/xmlschema-0/.

•     The XML Schema Structures (Part 1), which can be found at http://www.w3c.org/TR/xmlschema-1/.

•     The XML Schema Data Types (Part 2), which can be found at http://www.w3c.org/TR/xmlschema-2/.

You can find additional information on the XML Schema Definition Language at http://www.w3c.org/XML/Schema.

Sample XML Document

The rest of this chapter is devoted to creating and understanding the XML schema for the XML document shown in Listing 4.1, which details a purchase order for various items that can commonly be found in a grocery store. This document allows one individual to receive the shipment of the goods and an entirely different individual to pay for the pur-chase. This document also contains specific information about the products ordered, such as how much each product is, how many were ordered, and so on.

LISTING 4.1 PurchaseOrder.xml Contains a Sample Purchase Order for Common Items Found in a Grocery Store

<PurchaseOrder  Tax=”5.76”  Total=”75.77”>

<ShippingInformation> <Name>Dillon Larsen</Name> <Address>

<Street>123 Jones Rd.</Street> <City>Houston</City> <State>TX</State> <Zip>77381</Zip>

</Address>

<Method>USPS</Method>

LISTING 4.1 continued

<DeliveryDate>2001-08-12</DeliveryDate> </ShippingInformation>

<BillingInformation> <Name>Madi Larsen</Name> <Address>

<Street>123 Jones Rd.</Street> <City>Houston</City> <State>TX</State> <Zip>77381</Zip>

</Address>

<PaymentMethod>Credit Card</PaymentMethod> <BillingDate>2001-08-09</BillingDate>

</BillingInformation>

<Order  SubTotal=”70.01”  ItemsSold=”17”>

<Product  Name=”Baby  Swiss”  Id=”702890”  Price=”2.89”

➥ Quantity=”1”/>

<Product  Name=”Hard  Salami”  Id=”302340”  Price=”2.34”

➥ Quantity=”1”/>

<Product  Name=”Turkey”  Id=”905800”  Price=”5.80”

➥ Quantity=”1”/>

<Product  Name=”Caesar  Salad”  Id=”991687”  Price=”2.38”

➥ Quantity=”2”/>

<Product  Name=”Chicken  Strips”  Id=”133382”  Price=”2.50”

➥ Quantity=”1”/>

<Product  Name=”Bread”  Id=”298678”  Price=”1.08”

➥ Quantity=”1”/>

<Product  Name=”Rolls”  Id=”002399”  Price=”2.24”

➥ Quantity=”1”/>

<Product  Name=”Cereal”  Id=”066510”  Price=”2.18”

➥ Quantity=”1”/>

<Product  Name=”Jalapenos”  Id=”101005”  Price=”1.97”

➥ Quantity=”1”/>

<Product  Name=”Tuna”  Id=”000118”  Price=”0.92”

➥ Quantity=”3”/>

<Product  Name=”Mayonnaise”  Id=”126860”  Price=”1.98”

➥ Quantity=”1”/>

<Product  Name=”Top  Sirloin”  Id=”290502”  Price=”9.97”

➥ Quantity=”2”/>

<Product  Name=”Soup”  Id=”001254”  Price=”1.33”

➥ Quantity=”1”/>

<Product  Name=”Granola  Bar”  Id=”026460”  Price=”2.14”

➥ Quantity=”2”/>

<Product  Name=”Chocolate  Milk”  Id=”024620”  Price=”1.58”

➥ Quantity=”2”/>

<Product  Name=”Spaghetti”  Id=”000265”  Price=”1.98”

➥ Quantity=”1”/>

LISTING 4.1 continued

<Product  Name=”Laundry  Detergent”  Id=”148202”  Price=”8.82”

➥ Quantity=”1”/> </Order>

</PurchaseOrder>

As you can see, Listing 4.1 represents a fairly small and simple order that could be placed online. It contains the necessary information regarding how payment is to be made, how the order is to be shipped, and on what day the order is to be delivered. The preceding listing should by no means be construed as an all-inclusive document for an online grocery store order; it has been constructed for use as an example within this book only.

Until the XML Schema Definition Language recommendation was finalized, most authors, in the face of ever-changing standards, decided to stick with a finalized standard of DTDs. For the preceding listing, an author might construct the DTD shown in

Listing 4.2.

LISTING 4.2 PurchaseOrder.dtd Contains a Sample DTD for PurchaseOrder.xml

<?xml  version=”1.0”  encoding=”UTF-8”?>

<!ELEMENT PurchaseOrder (ShippingInformation, BillingInformation, Order)> <!ATTLIST PurchaseOrder

Tax CDATA #IMPLIED Total CDATA #IMPLIED

> 

<!ELEMENT  ShippingInformation  (Name,  Address,  (((BillingDate,

➥ PaymentMethod))  |  ((DeliveryDate,  Method))))>

<!ELEMENT  BillingInformation  (Name,  Address,  (((BillingDate,

➥ PaymentMethod)) | ((DeliveryDate, Method))))> <!ELEMENT Order (Product+)>

<!ATTLIST  Order

SubTotal  CDATA  #IMPLIED

ItemsSold  CDATA  #IMPLIED>

<!ELEMENT  Name  (#PCDATA)>

<!ELEMENT Address (Street, City, State, Zip)> <!ELEMENT BillingDate (#PCDATA)>

<!ELEMENT PaymentMethod (#PCDATA)> <!ELEMENT DeliveryDate (#PCDATA)> <!ELEMENT Method (#PCDATA)> <!ELEMENT Product EMPTY>

<!ATTLIST Product Name CDATA #IMPLIED Id CDATA #IMPLIED

LISTING 4.2 continued

Price CDATA #IMPLIED Quantity CDATA #IMPLIED

> 

<!ELEMENT Street (#PCDATA)> <!ELEMENT City (#PCDATA)> <!ELEMENT State (#PCDATA)> <!ELEMENT Zip (#PCDATA)>

Schema for XML Document

So, now that you’ve seen the DTD for the XML document in Listing 4.1, what would the comparative XML schema for it look like? Although the DTD in Listing 4.2 manages to describe the XML document in Listing 4.1 in a total of 30 lines, creating an XML schema is not quite so easy. Given the document in Listing 4.1, the XML schema for it is shown in Listing 4.3.

LISTING 4.3 PurchaseOrder.xsd Contains the Schema Definition for

PurchaseOrder.xml

<xsd:schema  xmlns:xsd=”http://www.w3.org/2001/XMLSchema”>

<xsd:annotation>

<xsd:documentation>

Purchase Order schema for an online grocery store. </xsd:documentation>

</xsd:annotation>

<xsd:element  name=”PurchaseOrder”  type=”PurchaseOrderType”/>

<xsd:complexType name=”PurchaseOrderType”> <xsd:all>

<xsd:element name=”ShippingInformation” type=”InfoType” ➥ minOccurs=”1” maxOccurs=”1”/>

<xsd:element name=”BillingInformation” type=”InfoType” ➥ minOccurs=”1” maxOccurs=”1”/>

<xsd:element name=”Order” type=”OrderType” ➥ minOccurs=”1” maxOccurs=”1”/>

</xsd:all>

<xsd:attribute name=”Tax”> <xsd:simpleType>

<xsd:restriction base=”xsd:decimal”> <xsd:fractionDigits value=”2”/>

</xsd:restriction>

</xsd:simpleType>

</xsd:attribute> <xsd:attribute name=”Total”>

<xsd:simpleType>

<xsd:restriction base=”xsd:decimal”> <xsd:fractionDigits value=”2”/>

</xsd:restriction>

</xsd:simpleType>

</xsd:attribute>

</xsd:complexType>

<xsd:group name=”ShippingInfoGroup”> <xsd:all>

<xsd:element name=”DeliveryDate” type=”DateType”/> <xsd:element name=”Method” type=”DeliveryMethodType”/>

</xsd:all>

</xsd:group>

<xsd:group name=”BillingInfoGroup”> <xsd:all>

<xsd:element name=”BillingDate” type=”DateType”/> <xsd:element name=”PaymentMethod” type=”PaymentMethodType”/>

</xsd:all>

</xsd:group>

<xsd:complexType name=”InfoType”> <xsd:sequence>

<xsd:element name=”Name” minOccurs=”1” maxOccurs=”1”> <xsd:simpleType>

<xsd:restriction base=”xsd:string”/> </xsd:simpleType>

</xsd:element>

<xsd:element name=”Address” type=”AddressType” minOccurs=”1” ➥ maxOccurs=”1”/>

<xsd:choice minOccurs=”1” maxOccurs=”1”> <xsd:group ref=”BillingInfoGroup”/> <xsd:group ref=”ShippingInfoGroup”/>

</xsd:choice>

</xsd:sequence>

</xsd:complexType>

<xsd:simpleType name=”DateType”> <xsd:restriction base=”xsd:date”/>

</xsd:simpleType>

<xsd:simpleType name=”DeliveryMethodType”> <xsd:restriction base=”xsd:string”>

<xsd:enumeration value=”USPS”/>

<xsd:enumeration value=”UPS”/>

<xsd:enumeration value=”FedEx”/>

<xsd:enumeration value=”DHL”/>

<xsd:enumeration value=”Other”/>

</xsd:restriction>

</xsd:simpleType>

<xsd:simpleType name=”PaymentMethodType”> <xsd:restriction base=”xsd:string”>

<xsd:enumeration value=”Check”/> <xsd:enumeration value=”Cash”/> <xsd:enumeration value=”Credit Card”/>

<xsd:enumeration value=”Debit Card”/> <xsd:enumeration value=”Other”/>

</xsd:restriction>

</xsd:simpleType>

<xsd:complexType name=”AddressType”> <xsd:all>

<xsd:element name=”Street” minOccurs=”1”> <xsd:simpleType>

<xsd:restriction base=”xsd:string”/> </xsd:simpleType>

</xsd:element>

<xsd:element name=”City” minOccurs=”1” maxOccurs=”1”> <xsd:simpleType>

<xsd:restriction base=”xsd:string”/> </xsd:simpleType>

</xsd:element>

<xsd:element name=”State” type=”StateType” minOccurs=”1” ➥ maxOccurs=”1”/>

<xsd:element name=”Zip” type=”ZipType” minOccurs=”1” ➥ maxOccurs=”1”/>

</xsd:all>

</xsd:complexType>

<xsd:simpleType name=”ZipType”> <xsd:restriction base=”xsd:string”>

<xsd:minLength value=”5”/> <xsd:maxLength value=”10”/>

<xsd:pattern value=”[0-9]{5}(-[0-9]{4})?”/> </xsd:restriction>

</xsd:simpleType>

<xsd:simpleType name=”StateType”> <xsd:restriction base=”xsd:string”>

<xsd:length value=”2”/>

 <xsd:enumeration value=”AR”/>

<xsd:enumeration value=”LA”/>

<xsd:enumeration value=”MS”/>

<xsd:enumeration value=”OK”/>

<xsd:enumeration value=”TX”/>

</xsd:restriction>

</xsd:simpleType>

<xsd:complexType name=”OrderType”> <xsd:sequence>

<xsd:element name=”Product” type=”ProductType” ➥ minOccurs=”1” maxOccurs=”unbounded”/>

</xsd:sequence>

<xsd:attribute name=”SubTotal”> <xsd:simpleType>

<xsd:restriction base=”xsd:decimal”> <xsd:fractionDigits value=”2”/>

</xsd:restriction>

</xsd:simpleType>

</xsd:attribute>

<xsd:attribute name=”ItemsSold” type=”xsd:positiveInteger”/> </xsd:complexType>

<xsd:complexType name=”ProductType”> <xsd:attribute name=”Name” type=”xsd:string”/>

<xsd:attribute name=”Id” type=”xsd:positiveInteger”/> <xsd:attribute name=”Price”>

<xsd:simpleType>

<xsd:restriction base=”xsd:decimal”> <xsd:fractionDigits value=”2”/>

</xsd:restriction>

</xsd:simpleType>

</xsd:attribute>

<xsd:attribute name=”Quantity” type=”xsd:positiveInteger”/> </xsd:complexType>

</xsd:schema>

Examining the preceding XML schema, you can see that defining a schema for a docu-ment can become fairly complicated. However, for all the extra complexity involved, the schema gives the author virtually limitless control over how an XML document can be validated against it. For instance, you may notice the use of the <xsd:choice> element. You’ll learn later in the “Model Groups” section of this chapter that this element can be used to indicate when one of a group of elements or attributes may show up, but not all, as is the case with the DeliveryDate and BillingDate attributes.

Also, notice the use of the xsd namespace. This namespace can be anything, but for con-vention in this chapter, we’ll use xsd to indicate an XML Schema Definition Language element.