XML Schema Complex Elements and Data Types

Define XML Schema complex elements and data types with XSD through focused document structure examples.

How do we define complex elements in XML Schema?

<book>
<title>Yunus Emre Divani</title>
<language>Turkish</language>
</book>

There are two common ways to define complex elements.

<xs:element name="book">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="title" type="xs:string"/>
      <xs:element name="language" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

In the first method, we define the complex type directly inside the element.

If the structure is unique to that element and will not be reused elsewhere, this approach is fine.

If the same structure will be used by multiple elements, I recommend the second method:

<xs:element name="book" type="bookEntry"/>
<xs:element name="aboutBook" type="bookEntry"/>
<!-- You can define for one or more elements. -->
  <xs:complexType name="bookEntry">
    <xs:sequence>
      <xs:element name="title" type="xs:string"/>
      <xs:element name="language" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

You can also extend a complex type with the properties of another type. From this point on, I will generally use the second method in the examples.

<xs:element name="book" type="bookEntry"/>
<xs:element name="aboutBook" type="aboutBook"/>
<!-- You can define for one or more elements. -->
  <xs:complexType name="bookEntry">
    <xs:sequence>
      <xs:element name="title" type="xs:string"/>
      <xs:element name="language" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

<xs:complexType name="aboutBook">
  <xs:complexContent>
    <xs:extension base="bookEntry">
      <xs:sequence>
        <xs:element name="author" type="xs:string"/>
        <xs:element name="isbn" type="xs:string"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

Empty Elements

<book isbn="9759954949"/>

To define this as a complex element:

<xs:element name="book" type="bookEntry"/>

<xs:complexType name="bookEntry">
  <xs:attribute name="isbn" type="xs:positiveInteger"/>
</xs:complexType>

This defines an empty book element with only an isbn attribute.

Element Only

<book>
  <title>Yunus Emre Divani</title>
  <language>Turkish</language>
</book>

For this XML example, the complex type definition looks like this:

<xs:element name="book" type="bookStructure"/>

<xs:complexType name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

Text Content Only

<book isbn="9759954949">Yunus Emre Divani</book>

To define a book element that contains text and also has an attribute:

<xs:element name="book" type="bookStructure"/>

<xs:complexType name="bookStructure">
  <xs:simpleContent>
    <xs:extension base="xs:string">
      <xs:attribute name="isbn" type="xs:positiveInteger" />
    </xs:extension>
  </xs:simpleContent>
</xs:complexType>

Mixed Content

<book>
  Book Title: <title>Yunus Emre Divani</title>
  Language: <language>Turkish</language>
  ISBN: <isbn>9759954949</isbn>
</book>

For elements with mixed content, define the complex type as follows:

<xs:element name="book" type="bookStructure"/>

<xs:complexType mixed="true" name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
    <xs:element name="isbn" type="xs:positiveInteger"/>
  </xs:sequence>
</xs:complexType>

Indicators

<all> allows child elements to appear in any order. Each child can appear at most once unless occurrence rules are defined.

<xs:complexType mixed="true" name="bookStructure">
  <xs:all>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
    <xs:element name="isbn" type="xs:positiveInteger"/>
  </xs:all>
</xs:complexType>

<choice> allows one of the listed child elements to be used.

<xs:complexType mixed="true" name="bookStructure">
  <xs:choice>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
    <xs:element name="isbn" type="xs:positiveInteger"/>
  </xs:choice>
</xs:complexType>

<sequence> requires child elements to appear in the specified order.

<xs:complexType mixed="true" name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
    <xs:element name="isbn" type="xs:positiveInteger"/>
  </xs:sequence>
</xs:complexType>

<maxOccurs> determines the maximum number of times a child element can appear. <minOccurs> determines the minimum number of times a child element must appear.

<xs:complexType mixed="true" name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string" maxOccurs="10" minOccurs="1"/>
  </xs:sequence>
</xs:complexType>

In the example above, the language element must appear at least once and can appear at most 10 times.

With <group>, you can create a reusable group and use it in multiple elements. You can also use the indicators above inside a group.

<xs:group name="bookGroup">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
  </xs:sequence>
</xs:group>

<xs:element name="book" type="bookEntry"/>

<xs:complexType name="bookEntry">
  <xs:sequence>
    <xs:group ref="bookGroup"/>
    <xs:element name="isbn" type="xs:positiveInteger"/>
  </xs:sequence>
</xs:complexType>

The same idea applies to <attributeGroup>:

<xs:attributeGroup name="bookAttribute">
  <xs:attribute name="isbn" type="xs:positiveInteger"/>
</xs:attributeGroup>

<xs:element name="book" type="bookEntry"/>

<xs:complexType name="bookEntry">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
  </xs:sequence>
  <xs:attributeGroup ref="bookAttribute"/>
</xs:complexType>

<any> is used to allow extension elements. In the example below, another element can be added after the defined fields.

<xs:complexType name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
    <xs:any minOccurs="0"/>
  </xs:sequence>
</xs:complexType>

<anyAttribute> is used to allow additional attributes.

<xs:complexType name="bookStructure">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="language" type="xs:string"/>
  </xs:sequence>
  <xs:anyAttribute/>
</xs:complexType>

substitutionGroup lets one element substitute for another element.

<xs:element name="book" type="xs:string"/>
<xs:element name="title" substitutionGroup="book"/>
<!-- When defining XML -->
<book>Yunus Emre Divani</book>
<!-- or -->
<title>Yunus Emre Divani</title>

This lets us use either element in the XML. If we add block="substitution" as an attribute, the <title></title> form shown above is no longer allowed.

Data Types

Now let's look at XML Schema data types.

Text Data Types

Data Type	Description
ENTITIES	A list of entity references
ENTITY	An entity reference
ID	A unique ID value assigned to an element
IDREF	A reference to another element's ID
IDREFS	A list of ID references
language	A valid language identifier
Name	A valid XML name
NCName	An XML name without a namespace prefix
NMTOKEN	A valid XML name token
NMTOKENS	A list of XML name tokens
normalizedString	Text with normalized line breaks and tabs
QName	A qualified XML name
string	Text
token	Text with collapsed whitespace

Date Data Types

Data Type	Description
date	Date data
dateTime	Date and time data
duration	Interval data
gDay	Day (DD)
gMonth	Month (MM)
gMonthDay	Month - Day (MM-DD)
gYear	Year (YYYY)
gYearMonth	Year - Month (YYYY-MM)
time	Time

Numeric Data Types

Data Type	Description
byte	signed 8-bit integer
decimal	decimal value
int	signed 32-bit integer
integer	integer value
long	signed 64-bit integer
negativeInteger	Negative values (..,-2,-1)
nonNegativeInteger	Non-negative values (0,1,2,..)
nonPositiveInteger	Non-positive values (..,-2,-1,0)
positiveInteger	Positive values (1,2,..)
short	signed 16-bit integer
unsignedLong	unsigned 64-bit integer
unsignedInt	unsigned 32-bit integer
unsignedShort	unsigned 16-bit integer
unsignedByte	unsigned 8-bit integer

Miscellaneous Data Types

Data Type	Description
anyURI	URI value
base64Binary	Base64-encoded binary data
boolean	True or false value
double	Double-precision floating-point number
float	Floating-point number
hexBinary	Hexadecimal binary data
NOTATION	Notation declared in the schema
QName	Qualified XML name