<schema xmlns="http://www.w3.org/2001/XMLSchema">
<!-- schema content goes here -->
</schema>

The XSD namespace has changed quite a bit as the XSD specification itself has evolved. In the final specification, the XSD namespace is http://www.w3.org/2001/XMLSchema. If you come across an XSD schema that’s using a different namspace, you’ll know it’s written to an older (possibly draft) version of the XSD standard.

Within a schema, you define data types and declare elements. For example, assume you have this simple XML document:

<top>
  <child1>text</child1>
  <child2>text</child2>
</top>

The schema would first define a type that describes the contents of the <top> element. It would then declare and element of that type:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:complexType name=" topType">
  <xsd:sequence>
    <xsd:element name="child1" type="xsd:string"/>
    <xsd:element name="child2" type="xsd:string"/>
  </xsd:sequence>
</xsd:complexType>
<xsd:element name=" top" type=" topType"/>
</schema>

<xsd:complexType> defines a data type that contains child elements and/or attributes. The <top> element contains child element so it must be a complexType. Within this complex type, we specify that there will be a sequence of child1 and child2 elements. <xsd:sequence> means the elements must appear in that order, so if the XML document had <child2> first then <child1> that would violate the sequence and would be a validation error. Using <xsd:element> we declare each of the <child1> and <child2> elements and specify their data type as xsd:string. Note the use of the xsd: namespace prefix to indicate that this is one of the XSD built-in data types.

Now that we have defined the data type that describes our <top> element, we can declare the element itself using <xsd:element>. Note that we specify its type as topType which is the name we used for the complexType.

The basics of creating XSD schemas are: First define your data types then declare elements of those types. Lets take a look at some real-world examples.