XML schema

An XML schema is a mechanism, defined by the W3C, for describing and constraining the structure and content of XML documents.

Through its support for datatypes and namespaces, an XML schema has the potential to provide the standard structure for XML elements and attributes. Therefore, an XML schema, which is itself expressed in XML, can effectively define a class of XML documents of a given type, for example, stock item.

The following sample XML document describes an item for stock keeping purposes:

      '<?xml version="1.0" standalone="yes"?>'
   || '<!--Document for stock keeping example-->'
   || '<stockItem itemNumber="453-SR">'
   || '<itemName>Stainless steel rope thimbles</itemName>'
   || '<quantityOnHand>23</quantityOnHand>'
   || '<stockItem>';

The stock keeping example document is both well formed and valid according to the following schema called stock.xsd. (The numbers that precede each line are not part of the schema, but are used in the explanation after the schema.)

1. <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
2.
3. <xsd:element name="stockItem" type="stockItemType"/>
4. 
5. <xsd:complexType name="stockItemType">
6.  <xsd:sequence>
7.   <xsd:element name="itemName" type="xsd:string" minOccurs="0"/>
8.   <xsd:element name="quantityOnHand">
9.    <xsd:simpleType>
10.    <xsd:restriction base="xsd:nonNegativeInteger">
11.      <xsd:maxExclusive value="100"/>
12.     </xsd:restriction>
13.    </xsd:simpleType>
14.   </xsd:element>
15.  </xsd:sequence>
16.  <xsd:attribute name="itemNumber" type="SKU" use="required"/>
17. </xsd:complexType>
18.
19. <xsd:simpleType name="SKU">
20.  <xsd:restriction base="xsd:string">
21.   <xsd:pattern value="\d{3}-[A-Z]{2}"/>
22.  </xsd:restriction>
23. </xsd:simpleType>
24.
25. </xsd:schema>
The schema declares (line 3) that the root element is stockItem, which has a mandatory itemNumber attribute (line 16) of type SKU, and includes a sequence (lines 6 - 15) of other elements:
  • An optional itemName element of type string (line 7)
  • A required quantityOnHand element that has a constrained range of 1 - 99 based on the type nonNegativeInteger (lines 8 - 14)

Type declarations can be inline and unnamed, as in lines 9 - 13, which includes the maxExclusive facet to specify the legal values for the quantityOnHand element.

For the itemNumber attribute, by contrast, the named type SKU is declared separately in lines 19 - 23, which includes a pattern facet that uses regular expression syntax to specify that the legal values for that type consist of (in order) 3 digits, a hyphen-minus, and then two uppercase letters.