1 Introduction
This document, XML Schema Part 0: Primer, provides an
easily approachable description of the XML Schema
definition language, and should be used alongside the
formal descriptions of the language contained in Parts
1 and 2
of the XML Schema specification. The intended audience of
this document includes application developers whose
programs read and write schema documents, and schema
authors who need to know about the features of the
language, especially features that provide functionality
above and beyond what is provided by DTDs. The text assumes
that you have a basic understanding of XML 1.0 and ↑Namespaces in XML↑↓XML-Namespaces↓.
Each major section of the primer introduces new features of
the language, and describes those features in the context
of concrete examples.
Basic Concepts: The Purchase Order (§2) covers the basic mechanisms of
XML Schema. It describes how to declare the elements and
attributes that appear in XML documents, the distinctions
between simple and complex types, defining complex types,
the use of simple types for element and attribute values,
schema annotation, a simple mechanism for re-using element
and attribute definitions, and nil values.
Advanced Concepts I: Namespaces, Schemas & Qualification (§3), the first advanced section in
the primer, explains the basics of how namespaces are used
in XML and schema documents. This section is important for
understanding many of the topics that appear in the other
advanced sections.
Advanced Concepts II: The International Purchase Order (§4), the second advanced section
in the primer, describes mechanisms for deriving types from
existing types, and for controlling these derivations. The
section also describes mechanisms for merging together
fragments of a schema from multiple sources, and for
element substitution.
Advanced Concepts III: The Quarterly Report (§5) covers more
advanced features, including a mechanism for specifying
uniqueness among attributes and elements, a mechanism for
using types across namespaces, a mechanism for extending
types based on namespaces, and a description of how
documents are checked for conformance.
In addition to the sections just described, the primer
contains a number of appendices that
provide detailed reference information on simple types and
a regular expression language.
The primer is a non-normative document, which means that
it does not provide a definitive (from the W3C's point of
view) specification of the XML Schema language. The
examples and other explanatory material in this document
are provided to help you understand XML Schema, but they
may not always provide definitive answers. In such cases,
you will need to refer to the XML Schema specification, and
to help you do this, we provide many links pointing to the
relevant parts of the specification. More specifically, XML
Schema items mentioned in the primer text are linked to an
index [Index (§E)] of element names and attributes,
and a summary table of
datatypes, both in the primer. The table and the index
contain links to the relevant sections of XML Schema parts
1 and 2.
2 Basic Concepts: The Purchase Order
The purpose of a schema is to define a class of XML
documents, and so the term "instance document" is often
used to describe an XML document that conforms to a
particular schema. In fact, neither instances nor schemas
need to exist as documents per se -- they may
exist as streams of bytes sent between applications, as
fields in a database record, or as collections of XML
Infoset "Information Items" -- but to simplify the primer,
we have chosen to always refer to instances and schemas as
if they are documents and files.
Let us start by considering an instance document in a file
called po.xml. It
describes a purchase order generated by a home products
ordering and billing application:
The Purchase Order, po.xml
<?xml version="1.0"?>
<purchaseOrder orderDate="1999-10-20">
<shipTo country="US">
<name>Alice Smith</name>
<street>123 Maple Street</street>
<city>Mill Valley</city>
<state>CA</state>
<zip>90952</zip>
</shipTo>
<billTo country="US">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<city>Old Town</city>
<state>PA</state>
<zip>95819</zip>
</billTo>
<comment>Hurry, my lawn is going wild<!/comment>
<items>
<item partNum="872-AA">
<productName>Lawnmower</productName>
<quantity>1</quantity>
<USPrice>148.95</USPrice>
<comment>Confirm this is electric</comment>
</item>
<item partNum="926-AA">
<productName>Baby Monitor</productName>
<quantity>1</quantity>
<USPrice>39.98</USPrice>
<shipDate>1999-05-21</shipDate>
</item>
</items>
</purchaseOrder>
The purchase order consists of a main element,
purchaseOrder, and the subelements
shipTo, billTo, comment,
and items. These subelements (except
comment) in turn contain other subelements, and so
on, until a subelement such as USPrice
contains a number rather than any subelements. Elements
that contain subelements or carry attributes are said to
have complex types, whereas elements that contain numbers
(and strings, and dates, etc.) but do not contain any
subelements are said to have simple types. Some elements
have attributes; attributes always have simple types.
The complex types in the instance document, and some of
the simple types, are defined in the schema for purchase
orders. The other simple types are defined as part of XML
Schema's repertoire of built-in simple types.
Before going on to examine the purchase order schema, we
digress briefly to mention the association between the
instance document and the purchase order schema. As you can
see by inspecting the instance document, the purchase order
schema is not mentioned. An instance is not actually
required to reference a schema, and although many will, we
have chosen to keep this first section simple, and to
assume that any processor of the instance document can
obtain the purchase order schema without any information
from the instance document. In later sections, we will
introduce explicit mechanisms for associating instances and
schemas.
2.1 The Purchase Order Schema
The purchase order schema is contained in the file
po.xsd:
The Purchase Order Schema, po.xsd
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:annotation>
<xsd:documentation xml:lang="en">
Purchase order schema for Example.com.
Copyright 2000 Example.com. All rights reserved.
</xsd:documentation>
</xsd:annotation>
<xsd:element name="purchaseOrder" type="PurchaseOrderType"/>
<xsd:element name="comment" type="xsd:string"/>
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items"/>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
<xsd:complexType name="USAddress">
<xsd:sequence>
<xsd:element name="name" type="xsd:string"/>
<xsd:element name="street" type="xsd:string"/>
<xsd:element name="city" type="xsd:string"/>
<xsd:element name="state" type="xsd:string"/>
<xsd:element name="zip" type="xsd:decimal"/>
</xsd:sequence>
<xsd:attribute name="country" type="xsd:NMTOKEN"
fixed="US"/>
</xsd:complexType>
<xsd:complexType name="Items">
<xsd:sequence>
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU" use="required"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
<!-- Stock Keeping Unit, a code for identifying products -->
<xsd:simpleType name="SKU">
<xsd:restriction base="xsd:string">
<xsd:pattern value="\d{3}-[A-Z]{2}"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:schema>
The purchase order schema consists of a schema element and a variety
of subelements, most notably element, complexType, and simpleType which
determine the appearance of elements and their content in
instance documents.
Each of the elements in the schema has
a prefix xsd: which is associated with the XML
Schema namespace through the declaration,
xmlns:xsd="http://www.w3.org/2001/XMLSchema",
that appears in the
schema element. The prefix xsd: is
used by convention to denote the XML Schema namespace,
although any prefix can be used. The same prefix, and hence
the same association, also appears on the names of built-in
simple types, e.g. xsd:string. The purpose of the association
is to identify the elements and simple types as belonging
to the vocabulary of the XML Schema language rather than
the vocabulary of the schema author. For the sake of
clarity in the text, we just mention the names of elements
and simple types (e.g.
simpleType), and omit the prefix.
2.2 Complex Type Definitions, Element & Attribute Declarations
In XML Schema, there is a basic difference between complex
types which allow elements in their content and may carry
attributes, and simple types which cannot have element
content and cannot carry attributes. There is also a major
distinction between definitions which create new types
(both simple and complex), and declarations which enable
elements and attributes with specific names and types (both
simple and complex) to appear in document instances. In
this section, we focus on defining complex types and
declaring the elements and attributes that appear within
them.
New complex types are defined using
the
complexType element and such definitions
typically contain a set of element declarations, element
references, and attribute declarations. The declarations
are not themselves types, but rather an association between
a name and the constraints which govern the appearance of that
name in documents governed by the associated schema.
Elements are declared using the element element, and
attributes are declared using the attribute element. For
example, USAddress is defined as a complex
type, and within the definition of USAddress
we see five element declarations and one attribute
declaration:
Defining the USAddress Type
<xsd:complexType name="USAddress" >
<xsd:sequence>
<xsd:element name="name" type="xsd:string"/>
<xsd:element name="street" type="xsd:string"/>
<xsd:element name="city" type="xsd:string"/>
<xsd:element name="state" type="xsd:string"/>
<xsd:element name="zip" type="xsd:decimal"/>
</xsd:sequence>
<xsd:attribute name="country" type="xsd:NMTOKEN" fixed="US"/>
</xsd:complexType>
The consequence of this definition is
that any element appearing in an instance whose type is
declared to be USAddress (e.g.
shipTo in
po.xml) must consist of five elements and one
attribute. These elements must be called name,
street, city, state
and zip as specified by the values of the
declarations' name attributes, and the
elements must appear in the same sequence (order) in which
they are declared. The first four of these elements will
each contain a string, and the fifth will contain a
number. The element whose type is declared to be
USAddress may appear with an attribute called
country which must contain the string
US.
The USAddress definition
contains only declarations involving the simple types: string,
decimal and
NMTOKEN. In contrast, the
PurchaseOrderType definition contains element
declarations involving complex types, e.g.
USAddress, although note that both declarations use
the same type
attribute to identify the type, regardless of whether the
type is simple or complex.
Defining PurchaseOrderType
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items"/>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
In defining PurchaseOrderType, two of the
element declarations, for shipTo and
billTo, associate different element names with the
same complex type, namely USAddress. The
consequence of this definition is that any element
appearing in an instance document (e.g.
po.xml) whose type is declared to be
PurchaseOrderType must consist of elements named
shipTo and billTo, each
containing the five subelements (name,
street, city, state and
zip) that were declared as part of
USAddress. The shipTo and
billTo elements may also carry the
country attribute that was declared as part of
USAddress.
The PurchaseOrderType definition contains an
orderDate attribute declaration which, like
the country attribute declaration, identifies
a simple type. In fact, all attribute declarations must
reference simple types because, unlike element
declarations, attributes cannot contain other elements or
other attributes.
The element declarations we have
described so far have each associated a name with an
existing type definition. Sometimes it is preferable to use
an existing element rather than declare a new element, for
example:
<xsd:element ref="comment" minOccurs="0"/>
This declaration references an existing element,
comment, that was declared elsewhere in the purchase
order schema. In general, the value of the ref attribute must reference a
global element, i.e. one that has been declared under
schema rather
than as part of a complex type definition. The consequence
of this declaration is that an element called
comment may appear in an instance document, and its
content must be consistent with that element's type, in
this case, string.
2.2.1 Occurrence Constraints
The comment element is
optional within PurchaseOrderType because the
value of the
minOccurs attribute in its declaration is 0. In
general, an element is required to appear when the value of
minOccurs
is 1 or more. The maximum number of times an element may
appear is determined by the value of a maxOccurs attribute in
its declaration. This value may be a positive integer such
as 41, or the term unbounded to indicate there
is no maximum number of occurrences. The default value for
both the
minOccurs and the maxOccurs attributes is
1. Thus, when an element such as comment is
declared without a
maxOccurs attribute, the element may not occur
more than once. Be sure that if you specify a value for
only the
minOccurs attribute, it is less than or equal to
the default value of
maxOccurs, i.e. it is 0 or 1. Similarly, if you
specify a value for only the maxOccurs attribute, it
must be greater than or equal to the default value of
minOccurs,
i.e. 1 or more. If both attributes are omitted, the element
must appear exactly once.
Attributes may appear once or not at
all, but no other number of times, and so the
syntax for specifying
occurrences of attributes is different than the syntax for
elements. In particular, attributes can be declared with a
use attribute
to indicate whether the attribute is
required (see for example, the partNum
attribute declaration in
po.xsd), optional, or even
prohibited.
Default values of both attributes and elements are declared using the
default attribute, although this attribute has a slightly
different consequence in each case. When an attribute is declared with a
default value, the value of the attribute is whatever value
appears as the attribute's value in an instance document; if
the attribute does not appear in the instance document, the schema
processor provides the attribute with a value equal to that of the
default attribute.
Note that default values for attributes only make sense if the
attributes themselves are optional, and so it is an error to specify
both a default value and anything other than a value of
optional for use.
The schema processor treats defaulted elements slightly differently.
When an element is declared with a default value, the value of the
element is whatever value appears as the element's content in the
instance document; if the element appears without any content, the
schema processor provides the element with a value equal to that of the
default attribute.
However, if the element does not appear in the instance document, the schema
processor does not provide the element at all. In summary, the
differences between element and attribute defaults can be stated as:
Default attribute values apply when attributes are missing, and default
element values apply when elements are empty.
The fixed attribute is used in both attribute and element
declarations to ensure that the attributes and elements are
set to particular values. For example, po.xsd
contains a declaration for the country
attribute, which is declared with a fixed value US. This
declaration means that the appearance of a country
attribute in an instance document is optional (the default value of
use is optional),
although if the attribute does appear, its value must be US,
and if the attribute does not appear, the schema processor will provide
a country attribute with the value US. Note
that the concepts of a fixed value and a default value are mutually
exclusive, and so it is an error for a declaration to contain both
fixed and default attributes.
The values of the attributes used in element and attribute
declarations to constrain their occurrences are summarized in
Table 1.
| Table 1. Occurrence Constraints for Elements and Attributes |
|---|
|
|
| Notes |
|---|
| (1, 1) -, - | required, -, - | element/attribute must appear once, it may have any value |
| (1, 1) 37, - | required, 37, - | element/attribute must appear once, its value must be 37 |
| (2, unbounded) 37, - | n/a | element must appear twice or more, its value must be
37; in general,
minOccurs and
maxOccurs values may be positive integers, and
maxOccurs value
may also be "unbounded" |
| (0, 1) -, - | optional, -, - | element/attribute may appear once, it may have any value |
| ↑(0, 1) 37, -↑ | ↑n/a↑ | ↑element may appear once, if it does not
appear it is not provided; if it does appear and it is empty, its
value is 37; if it does appear and it is not empty,
its value must be 37↑ |
| ↓(0, 1) 37, -↓↑n/a↑ | optional, 37, - | ↓element/↓attribute may appear once, if it does appear
its value must be 37, if it does not appear its value is 37 |
| ↑(0, 1) -, 37↑ | ↑n/a↑ | ↑element may appear once; if it does not
appear it is not provided; if it does
appear and it is empty, its value is 37; otherwise its value is that
given↑ |
| ↓(0, 1) -, 37↓↑n/a↑ | optional, -, 37 | ↓element/↓attribute may appear once; if it does not
appear its value is 37, otherwise its value is that given |
| (0, 2) -, 37 | n/a | element may appear once, twice, or not at all; if the
element does not appear it is not provided; if it does
appear and it is empty, its value is 37; otherwise its
value is that given; in general, minOccurs and maxOccurs values may be
positive integers, and maxOccurs value may also
be "unbounded" |
| (0, 0) -, - | prohibited, -, - | element/attribute must not appear |
|
Note that neither
minOccurs,
maxOccurs, nor use
may appear in the declarations of global elements and
attributes. |
2.2.2 Global Elements & Attributes
Global elements, and global attributes, are created by
declarations that appear as the children of the schema element. Once
declared, a global element or a global attribute can be
referenced in one or more declarations using the ref attribute as described
above. A declaration that references a global element
enables the referenced element to appear in the instance
document in the context of the referencing declaration. So, for
example, the comment element appears in
po.xml at the same level as
the shipTo, billTo and
items elements because the declaration that
references comment appears in the complex type
definition at the same level as the declarations of the
other three elements.
The declaration of a global element also enables the
element to appear at the top-level of an instance document.
Hence purchaseOrder, which is declared as a
global element in
po.xsd, can appear as the top-level element in
po.xml. Note that this
rationale will also allow a comment element to
appear as the top-level element in a document like
po.xml.
There are a number of caveats concerning the use of
global elements and attributes. One caveat is that global
declarations cannot contain references; global declarations
must identify simple and complex types directly. Put
concretely, global declarations cannot contain the ref attribute, they must
use the type
attribute (or, as we describe shortly, be followed by an anonymous type definition). A
second caveat is that cardinality constraints cannot be
placed on global declarations, although they can be placed
on local declarations that reference global declarations.
In other words, global declarations cannot contain the
attributes minOccurs, maxOccurs, or use.
2.2.3 Naming Conflicts
We have now described how to define new complex types
(e.g. PurchaseOrderType), declare elements
(e.g. purchaseOrder) and declare attributes
(e.g. orderDate). These activities generally
involve naming, and so the question naturally arises: What
happens if we give two things the same name? The answer
depends upon the two things in question, although in
general the more similar are the two things, the more
likely there will be a conflict.
Here are some examples to illustrate when same names cause
problems. If the two things are both types, say we define a
complex type called USStates and a simple type called
USStates, there is a conflict. If the two things are a type
and an element or attribute, say we define a complex type
called USAddress and we declare an element called USAddress,
there is no conflict. If the two things are elements within
different types (i.e. not global elements), say we declare
one element called name as part of the USAddress type and a
second element called name as part of the Item type, there
is no conflict. (Such elements are sometimes called local
element declarations.) Finally, if the two things are both
types and you define one and XML Schema has defined the
other, say you define a simple type called decimal, there
is no conflict. The reason for the apparent contradiction
in the last example is that the two types belong to
different namespaces. We explore the use of namespaces
in schema in a later section.
2.3 Simple Types
The purchase order schema declares several elements and
attributes that have simple types. Some of these simple
types, such as string
and decimal, are built
in to XML Schema, while others are derived from the
built-in's. For example, the partNum attribute
has a type called SKU (Stock Keeping Unit)
that is derived from
string. Both built-in simple types and their
derivations can be used in all element and attribute
declarations. Table 2 lists
all the simple types built in to XML Schema, along with
examples of the different types.
| Table 2. Simple Types Built In to XML Schema |
|---|
| Simple Type | Examples (delimited by commas) | Notes |
|---|
| string | Confirm this is electric | |
|
normalizedString | Confirm this is electric | see (3) |
| token | Confirm this is electric | see (4) |
|
base64Binary | GpM7 | |
| hexBinary | 0FB7 | |
| integer | ↓-126789, -1, 0, 1, 126789↓↑...-1, 0, 1, ...↑ | see (2) |
| positiveInteger | ↓1, 126789↓↑1, 2, ...↑ | see (2) |
|
negativeInteger | ↓-126789, -1↓↑... -2, -1↑ | see (2) |
| nonNegativeInteger | ↓0, 1, 126789↓↑0, 1, 2, ...↑ | see (2) |
| nonPositiveInteger | ↓-126789, -1, 0↓↑... -2, -1, 0↑ | see (2) |
| long | ↓-1, 12678967543233↓↑-9223372036854775808, ... -1, 0, 1, ... 9223372036854775807↑ | see (2) |
| unsignedLong | ↓0, 12678967543233↓↑0, 1, ... 18446744073709551615↑ | see (2) |
| int | ↓-1, 126789675↓↑-2147483648, ... -1, 0, 1, ... 2147483647↑ | see (2) |
| unsignedInt | ↓0, 1267896754↓↑0, 1, ...4294967295↑ | see (2) |
| short | ↓-1, 12678↓↑-32768, ... -1, 0, 1, ... 32767↑ | see (2) |
| unsignedShort | ↓0, 12678↓↑0, 1, ... 65535↑ | see (2) |
| byte | ↓-1, 126↓↑-128, ...-1, 0, 1, ... 127↑ | see (2) |
| unsignedByte | ↓0, 126↓↑0, 1, ... 255↑ | see (2) |
| decimal | -1.23, 0, 123.4, 1000.00 | see (2) |
| float | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to single-precision 32-bit floating point, NaN is "not a number", see (2) |
| double | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to double-precision 64-bit floating point, see (2) |
| boolean | true, false, 1, 0 | |
| duration | P1Y2M3DT10H30M12.3S | 1 year, 2 months, 3 days, 10 hours, 30 minutes, and 12.3 seconds |
| dateTime | 1999-05-31T13:20:00.000-05:00 | May 31st 1999 at 1.20pm Eastern Standard Time which
is 5 hours behind Co-Ordinated Universal Time, see
(2) |
| date | 1999-05-31 | see (2) |
| time | 13:20:00.000, 13:20:00.000-05:00 | see (2) |
|
gYear | 1999 | 1999, see (2) (5) |
| gYearMonth | 1999-02 | the month of February 1999, regardless of the number of days, see (2) (5) |
| gMonth | --05↓--↓ | May, see (2) (5) |
| gMonthDay | --05-31 | every May 31st, see (2) (5) |
|
gDay | ---31 | the 31st day, see (2) (5) |
| Name | shipTo | XML 1.0 Name type |
| QName | po:USAddress | XML Namespace QName |
| NCName | USAddress | XML Namespace NCName, i.e. a QName without the prefix and colon |
| anyURI |
| http://www.example.com/, | | http://www.example.com/doc.html#ID5 |
| |
| language | en-GB, en-US, fr | valid values for xml:lang as defined in XML 1.0 |
| ID | | XML 1.0 ID attribute type, see (1) |
| IDREF | | XML 1.0 IDREF attribute type, see (1) |
| IDREFS | | XML 1.0 IDREFS attribute type, see (1) |
| ENTITY | | XML 1.0 ENTITY attribute type, see (1) |
| ENTITIES | | XML 1.0 ENTITIES attribute type, see (1) |
| NOTATION | | XML 1.0 NOTATION attribute type, see (1) |
| NMTOKEN |
| XML 1.0 NMTOKEN attribute type, see (1) |
| NMTOKENS |
| US UK, | | Brésil Canada Mexique |
| XML 1.0 NMTOKENS attribute type, i.e. a whitespace
separated list of NMTOKEN's, see (1) |
|
Notes: (1) To retain compatibility
between XML Schema and XML 1.0 DTDs, the simple types
ID, IDREF, IDREFS, ENTITY, ENTITIES, NOTATION,
NMTOKEN, NMTOKENS should only be used in attributes.
(2) A value of this type can be represented by more
than one lexical format, e.g. 100 and 1.0E2 are both
valid float formats representing "one hundred".
However, rules have been established for this type
that define a canonical lexical format, see
XML Schema Part 2. (3) Newline, tab and
carriage-return characters in a normalizedString type are
converted to space characters before schema
processing. (4) As normalizedString, and adjacent space
characters are collapsed to a single space character,
and leading and trailing spaces are removed. (5) The "g"
prefix signals time periods in the Gregorian ↓calender↓↑calendar↑. |
New simple types are defined by
deriving them from existing simple types (built-in's and
derived). In particular, we can derive a new simple type by
restricting an existing simple type, in other words, the
legal range of values for the new type are a subset of the
existing type's range of values. We use the simpleType element to
define and name the new simple type. We use the restriction element
to indicate the existing (base) type, and to identify the
"facets" that constrain the range of values. A complete
list of facets is provided in
Appendix B.
Suppose we wish to create a new type
of integer called myInteger whose range of
values is between 10000 and 99999 (inclusive). We base our
definition on the built-in simple type integer, whose range of values also
includes integers less than 10000 and greater than 99999.
To define myInteger, we restrict the range of
the integer base type
by employing two facets called minInclusive and
maxInclusive:
Defining myInteger, Range 10000-99999
<xsd:simpleType name="myInteger">
<xsd:restriction base="xsd:integer">
<xsd:minInclusive value="10000"/>
<xsd:maxInclusive value="99999"/>
</xsd:restriction>
</xsd:simpleType>
The example shows one particular combination of a base
type and two facets used to define myInteger,
but a look at the list of built-in simple types and their
facets (Appendix B) should
suggest other viable combinations.
The purchase order schema contains
another, more elaborate, example of a simple type
definition. A new simple type called SKU is
derived (by restriction) from the simple type string. Furthermore, we
constrain the values of SKU using a facet
called pattern
in conjunction with the regular expression
"\d{3}-[A-Z]{2}" that is read "three digits
followed by a hyphen followed by two upper-case ASCII
letters":
Defining the Simple Type "SKU"
<xsd:simpleType name="SKU">
<xsd:restriction base="xsd:string">
<xsd:pattern value="\d{3}-[A-Z]{2}"/>
</xsd:restriction>
</xsd:simpleType>
This regular expression language is described more fully
in Appendix D.
XML Schema defines ↑twelve↑↓fifteen↓ facets
which are listed in Appendix
B. Among these, the enumeration facet is
particularly useful and it can be used to constrain the
values of almost every simple type, except the boolean type. The enumeration facet limits
a simple type to a set of distinct values. For example, we
can use the
enumeration facet to define a new simple type
called USState, derived from string, whose value must be one of the
standard US state abbreviations:
Using the Enumeration Facet
<xsd:simpleType name="USState">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="AK"/>
<xsd:enumeration value="AL"/>
<xsd:enumeration value="AR"/>
<!-- and so on ... -->
</xsd:restriction>
</xsd:simpleType>
USState would be a good replacement for the
string type currently
used in the state element declaration. By
making this replacement, the legal values of a
state element, i.e. the state
subelements of billTo and
shipTo, would be limited to one of AK,
AL, AR, etc. Note that the
enumeration values specified for a particular type must be
unique.
2.3.1 List Types
XML Schema has the concept of a list type, in addition to
the so-called atomic types that constitute most of the
types listed in Table 2.
(Atomic types, list types, and the union types described in
the next section are collectively called simple types.) The
value of an atomic type is indivisible from XML Schema's
perspective. For example, the
NMTOKEN value US is indivisible in
the sense that no part of US, such as the
character "S", has any meaning by itself. In contrast, list
types are comprised of sequences of atomic types and
consequently the parts of a sequence (the "atoms")
themselves are meaningful. For example, NMTOKENS is a list type, and an
element of this type would be a white-space delimited list
of NMTOKEN's, such as
"US UK FR". XML Schema has three built-in list types, they
are NMTOKENS,
IDREFS, and ENTITIES.
In addition to using the built-in
list types, you can create new list types by derivation
from existing atomic types. (You cannot create list types
from existing list types, nor from complex types.) For
example, to create a list of myInteger's:
Creating a List of myInteger's
<xsd:simpleType name="listOfMyIntType">
<xsd:list itemType="myInteger"/>
</xsd:simpleType>
And an element in an instance document whose content
conforms to listOfMyIntType is:
<listOfMyInt>20003 15037 95977 95945</listOfMyInt>
Several facets can be applied to list
types: length,
minLength,
maxLength, ↑pattern, ↑
and
enumeration. For example, to define a list of
exactly six US states (SixUSStates), we first
define a new list type called USStateList from
USState, and then we derive
SixUSStates by restricting USStateList
to only six items:
List Type for Six US States
<xsd:simpleType name="USStateList">
<xsd:list itemType="USState"/>
</xsd:simpleType>
<xsd:simpleType name="SixUSStates">
<xsd:restriction base="USStateList">
<xsd:length value="6"/>
</xsd:restriction>
</xsd:simpleType>
Elements whose type is SixUSStates must have
six items, and each of the six items must be one of the
(atomic) values of the enumerated type
USState, for example:
<sixStates>PA NY CA NY LA AK</sixStates>
Note that it is possible to derive a list type from the
atomic type string.
However, a string may
contain white space, and white space delimits the items in
a list type, so you should be careful using
list types whose base type is
string. For example, suppose we have defined a
list type with a
length facet equal to 3, and base type string, then the following 3 item
list is legal:
But the following 3 "item" list is illegal:
Asie Europe Amérique Latine
Even though "Amérique Latine" may exist as a single
string outside of the list, when it is included in the
list, the whitespace between Amérique and Latine
effectively creates a fourth item, and so the latter
example will not conform to the 3-item list type.
2.3.2 Union Types
Atomic types and list types enable an
element or an attribute value to be one or more instances
of one atomic type. In contrast, a union type enables an
element or attribute value to be one or more instances of
one type drawn from the union of multiple atomic and list
types. To illustrate, we create a union type for
representing American states as singleton letter
abbreviations or lists of numeric codes. The
zipUnion union type is built from one atomic type and
one list type:
Union Type for Zip Codes
<xsd:simpleType name="zipUnion">
<xsd:union memberTypes="USState listOfMyIntType"/>
</xsd:simpleType>
When we define a union type, the memberTypes
attribute value is a list of all the types in the union.
Now, assuming we have declared an element called
zips of type zipUnion, valid instances
of the element are:
<zips>CA</zips>
<zips>95630 95977 95945</zips>
<zips>AK</zips>
Two facets,
pattern and enumeration, can be
applied to a union type.
2.4 Anonymous Type Definitions
Schemas can be constructed by defining sets of named types
such as PurchaseOrderType and then declaring
elements such as purchaseOrder that reference
the types using the
type= construction. This style of schema
construction is straightforward but it can be unwieldy,
especially if you define many types that are referenced
only once and contain very few constraints. In these cases,
a type can be more succinctly defined as an anonymous type
which saves the overhead of having to be named and
explicitly referenced.
The definition of the type Items in po.xsd contains two element
declarations that use anonymous types (item
and quantity). In general, you can identify
anonymous types by the lack of a type= in an element (or
attribute) declaration, and by the presence of an un-named
(simple or complex) type definition:
Two Anonymous Type Definitions
<xsd:complexType name="Items">
<xsd:sequence>
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU" use="required"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
In the case of the item element, it has an
anonymous complex type consisting of the elements
productName, quantity,
USPrice, comment, and
shipDate, and an attribute called
partNum. In the case of the quantity
element, it has an anonymous simple type derived from
↓integer↓↑
positiveInteger↑ whose value
ranges between 1 and 99.
2.5 Element Content
The purchase order schema has many examples of elements
containing other elements (e.g. items),
elements having attributes and containing other elements
(e.g. shipTo), and elements containing only a
simple type of value (e.g. USPrice). However,
we have not seen an element having attributes but
containing only a simple type of value, nor have we seen an
element that contains other elements mixed with character
content, nor have we seen an element that has no content at
all. In this section we'll examine these variations in the
content models of elements.
2.5.1 Complex Types from Simple Types
Let us first consider how to declare an element that has
an attribute and contains a simple value. In an instance
document, such an element might appear as:
<internationalPrice currency="EUR">423.46</internationalPrice>
The purchase order schema declares a USPrice
element that is a starting point:
<xsd:element name="USPrice" type="decimal"/>
Now, how do we add an attribute to
this element? As we have said before, simple types cannot
have attributes, and
decimal is a simple type. Therefore, we must
define a complex type to carry the attribute declaration.
We also want the content to be simple type decimal. So our original question
becomes: How do we define a complex type that is based on
the simple type
decimal? The answer is to derive a new
complex type from the simple type
decimal:
Deriving a Complex Type from a Simple Type
<xsd:element name="internationalPrice">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="xsd:decimal">
<xsd:attribute name="currency" type="xsd:string"/>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
We use the
complexType element to start the definition of a
new (anonymous) type. To indicate that the content model of
the new type contains only character data and no elements,
we use a
simpleContent element. Finally, we derive the
new type by extending the simple
decimal type. The extension consists of adding a
currency attribute using a standard attribute
declaration. (We cover type derivation in detail in Advanced Concepts II: The International Purchase Order (§4).) The
internationalPrice element declared in this way will
appear in an instance as shown in the example at the
beginning of this section.
2.5.2 Mixed Content
The construction of the purchase order schema may be
characterized as elements containing subelements, and the
deepest subelements contain character data. XML Schema also
provides for the construction of schemas where character
data can appear alongside subelements, and character data
is not confined to the deepest subelements.
To illustrate, consider the following snippet from a
customer letter that uses some of the same elements as the
purchase order:
Snippet of Customer Letter
<letterBody>
<salutation>Dear Mr.<name>Robert Smith</name>.</salutation>
Your order of <quantity>1</quantity> <productName>Baby
Monitor</productName> shipped from our warehouse on
<shipDate>1999-05-21</shipDate>. ....
</letterBody>
Notice the text appearing between elements and their child
elements. Specifically, text appears between the elements
salutation, quantity,
productName and shipDate which are all
children of letterBody, and text appears
around the element ↓name↓↑name
↑ which is the child of a child of
letterBody. The following snippet of a schema
declares letterBody:
Snippet of Schema for Customer Letter
<xsd:element name="letterBody">
<xsd:complexType mixed="true">
<xsd:sequence>
<xsd:element name="salutation">
<xsd:complexType mixed="true">
<xsd:sequence>
<xsd:element name="name" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="quantity" type="xsd:positiveInteger"/>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
<!-- etc. -->
</xsd:sequence>
</xsd:complexType>
</xsd:element>
The elements appearing in the customer letter are
declared, and their types are defined using the element and complexType element
constructions we have seen before. To enable character data
to appear between the child-elements of
letterBody, the
mixed attribute on the type definition is set to
true.
Note that the mixed model in XML Schema
differs fundamentally from the
mixed model in XML 1.0. Under the XML Schema
mixed model, the order and number of child elements
appearing in an instance must agree with the order and
number of child elements specified in the model. In
contrast, under the XML 1.0 mixed model, the order and
number of child elements appearing in an instance cannot be
constrained. In summary, XML Schema provides full validation of
mixed models in contrast to the partial schema validation
provided by XML 1.0.
2.5.3 Empty Content
Now suppose that we want the
internationalPrice element to convey both the unit
of currency and the price as attribute values rather than
as separate attribute and content values. For example:
<internationalPrice currency="EUR" value="423.46"/>
Such an element has no content at
all; its content model is empty. To define a type whose
content is empty, we essentially define a type that allows
only elements in its content, but we do not actually
declare any elements and so the type's content model is
empty:
An Empty Complex Type
<xsd:element name="internationalPrice">
<xsd:complexType>
<xsd:complexContent>
<xsd:restriction base="xsd:anyType">
<xsd:attribute name="currency" type="xsd:string"/>
<xsd:attribute name="value" type="xsd:decimal"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
In this example, we define an (anonymous) type having
complexContent, i.e. only elements. The
complexContent element signals that we intend to
restrict or extend the content model of a complex type, and the
restriction of anyType declares
two attributes but does not introduce any element content
(see Deriving Complex Types by Restriction (§4.4) for more
details on restriction). The
internationalPrice element declared in this way may
legitimately appear in an instance as shown in the example
above.
The preceding syntax for an empty-content element is
relatively verbose, and it is possible to declare the
internationalPrice element more compactly:
Shorthand for an Empty Complex Type
<xsd:element name="internationalPrice">
<xsd:complexType>
<xsd:attribute name="currency" type="xsd:string"/>
<xsd:attribute name="value" type="xsd:decimal"/>
</xsd:complexType>
</xsd:element>
This compact syntax works because a complex type defined
without any simpleContent or
complexContent is interpreted as shorthand for
complex content that restricts anyType.
2.5.4 anyType
The anyType represents an abstraction called
the
ur-type which is the base type from which all
simple and complex types are derived. An
anyType type does not constrain its content in any
way. It is possible to use anyType like other
types, for example:
<xsd:element name="anything" type="xsd:anyType"/>
The content of the element declared in this way is
unconstrained, so the element value may be 423.46, but it
may be any other sequence of characters as well, or indeed
a mixture of characters and elements. In fact,
anyType is the default type when none is specified,
so the above could also be written as follows:
<xsd:element name="anything"/>
If unconstrained element content is needed, for example in
the case of elements containing prose which requires embedded
markup to support internationalization, then the default
declaration or a slightly restricted form of it may be suitable.
The text type described in Any Element, Any Attribute (§5.5)
is an example of such a type that is suitable for such
purposes.
2.6 Annotations
XML Schema provides three elements
for annotating schemas for the benefit of both human
readers and applications. In the purchase order schema, we
put a basic schema description and copyright information
inside the
documentation element, which is the recommended
location for human readable material. We recommend you use
the xml:lang attribute with any
documentation
elements to indicate the language of the information. Alternatively,
you may indicate the language of all information in a schema by placing
an xml:lang attribute on the schema element.
The ↑appinfo↑↓appInfo↓ element, which we did not use in the
purchase order schema, can be used to provide information
for tools, stylesheets and other applications. An
interesting example using ↑appinfo↑↓appInfo↓ is a
schema
that describes
the simple types in XML Schema Part 2: Datatypes.
Information describing this schema, e.g. which facets are
applicable to particular simple types, is represented
inside ↑appinfo↑↓appInfo↓
elements, and this information was used by an application to automatically
generate text for the XML Schema Part 2 document.
Both documentation and
↑appinfo↑↓appInfo↓ appear
as subelements of
annotation, which may itself appear at the
beginning of most schema constructions. To illustrate, the
following example shows annotation elements
appearing at the beginning of an element declaration and a
complex type definition:
Annotations in Element Declaration & Complex Type Definition
<xsd:element name="internationalPrice">
<xsd:annotation>
<xsd:documentation xml:lang="en">
element declared with anonymous type
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:annotation>
<xsd:documentation xml:lang="en">
empty anonymous type with 2 attributes
</xsd:documentation>
</xsd:annotation>
<xsd:complexContent>
<xsd:restriction base="xsd:anyType">
<xsd:attribute name="currency" type="xsd:string"/>
<xsd:attribute name="value" type="xsd:decimal"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
The
annotation element may also appear at the
beginning of other schema constructions such as those
indicated by the elements
schema,
simpleType, and attribute.
2.7 Building Content Models
The definitions of complex types in the purchase order
schema all declare sequences of elements that must appear
in the instance document. The occurrence of individual
elements declared in the so-called content models of these
types may be optional, as indicated by a 0 value for the
attribute
minOccurs (e.g. in comment), or be
otherwise constrained depending upon the values of minOccurs and
maxOccurs. XML
Schema also provides constraints that apply to groups of
elements appearing in a content model. These constraints
mirror those available in XML 1.0 plus some additional
constraints. Note that the constraints do not apply to
attributes.
XML Schema enables groups of elements to be defined and
named, so that the elements can be used to build up the
content models of complex types (thus mimicking common
usage of parameter entities in XML 1.0). Un-named groups of
elements can also be defined, and along with elements in
named groups, they can be constrained to appear in the same
order (sequence) as they are declared. Alternatively, they
can be constrained so that only one of the elements may
appear in an instance.
To illustrate, we introduce two groups
into the PurchaseOrderType definition from the
purchase order schema so that purchase orders may contain
either separate shipping and billing addresses, or a single
address for those cases in which the shippee and billee are
co-located:
Nested Choice and Sequence Groups
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:choice>
<xsd:group ref="shipAndBill"/>
<xsd:element name="singleUSAddress" type="USAddress"/>
</xsd:choice>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items"/>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
<xsd:group id="shipAndBill">
<xsd:sequence>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
</xsd:sequence>
</xsd:group>
The choice
group element allows only one of its children to appear in
an instance. One child is an inner group element that references
the named group shipAndBill consisting of the
element sequence shipTo, billTo,
and the second child is a singleUSAddress.
Hence, in an instance document, the
purchaseOrder element must contain either a
shipTo element followed by a billTo
element or a singleUSAddress element. The
choice group is
followed by the comment and items
element declarations, and both the choice group and the element
declarations are children of a sequence group. The effect
of these various groups is that the address element(s) must
be followed by comment and items
elements in that order.
There exists a third option for
constraining elements in a group: All the elements in the
group may appear once or not at all, and they may appear in
any order. The all
group (which provides a simplified version of the SGML
&-Connector) is limited to the top-level of any content
model. Moreover, the group's children must all be
individual elements (no groups), and no element in the
content model may appear more than once, i.e. the
permissible values of
minOccurs and maxOccurs are 0 and 1.
For example, to allow the child elements of
purchaseOrder to appear in any order, we could
redefine PurchaseOrderType as:
An 'All' Group
<xsd:complexType name="PurchaseOrderType">
<xsd:all>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items"/>
</xsd:all>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
By this definition, a comment element may
optionally appear within purchaseOrder, and it
may appear before or after any shipTo,
billTo and items elements, but it can
appear only once. Moreover, the stipulations of an all group do not allow us to
declare an element such as comment outside the
group as a means of enabling it to appear more than once.
XML Schema stipulates that an
all group must appear as the sole child at the
top of a content model. In other words, the following is
illegal:
Illegal Example with an 'All' Group
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:all>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element name="items" type="Items"/>
</xsd:all>
<xsd:sequence>
<xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
Finally, named and un-named groups that appear in content
models (represented by
group and
choice,
sequence,
all respectively) may carry minOccurs and maxOccurs
attributes. By combining and nesting the various groups
provided by XML Schema, and by setting the values of
minOccurs and
maxOccurs,
it is possible to represent any content model expressible
with an XML 1.0 DTD. Furthermore, the all group provides additional
expressive power.
2.8 Attribute Groups
Suppose we want to provide more information about each
item in a purchase order, for example, each item's weight
and preferred shipping method. We can accomplish this by
adding weightKg and shipBy
attribute declarations to the item element's
(anonymous) type definition:
Adding Attributes to the Inline Type Definition
<xsd:element name="↓Item↓↑item↑" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU" use="required"/>
<!-- add weightKg and shipBy attributes -->
<xsd:attribute name="weightKg" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
Alternatively, we can create a named
attribute group containing all the desired attributes of an
item element, and reference this group by name
in the item element declaration:
Adding Attributes Using an Attribute Group
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<!-- attributeGroup replaces individual declarations -->
<xsd:attributeGroup ref="ItemDelivery"/>
</xsd:complexType>
</xsd:element>
<xsd:attributeGroup id="ItemDelivery">
<xsd:attribute name="partNum" type="SKU" use="required"/>
<xsd:attribute name="weightKg" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
</xsd:attributeGroup>
Using an attribute group in this way can improve the
readability of schemas, and facilitates updating schemas
because an attribute group can be defined and edited in one
place and referenced in multiple definitions and
declarations. These characteristics of attribute groups
make them similar to parameter entities in XML 1.0. Note
that an attribute group may contain other attribute groups.
Note also that both attribute declarations and attribute
group references must appear at the end of complex type
definitions.
2.9 Nil Values
One of the purchase order items listed in po.xml, the Lawnmower,
does not have a shipDate element. Within the
context of our scenario, the schema author may have
intended such absences to indicate items not
yet shipped. But in general, the absence of an element does
not have any particular meaning: It may indicate that the
information is unknown, or not applicable, or the element
may be absent for some other reason. Sometimes it is
desirable to represent an unshipped item,
unknown information, or inapplicable information
explicitly with an element, rather than by an absent
element. For example, it may be desirable to represent a
"null" value being sent to or from a relational database
with an element that is present. Such cases can be
represented using XML Schema's nil mechanism which enables
an element to appear with or without a non-nil value.
XML Schema's nil mechanism involves
an "out of band" nil signal. In other words, there is no
actual nil value that appears as element content, instead
there is an attribute to indicate that the element content
is nil. To illustrate, we modify the
shipDate element declaration so that nils can be
signalled:
<xsd:element name="shipDate" type="xsd:date" nillable="true"/>
And to explicitly represent that
shipDate has a nil value in the instance
document, we set the nil
attribute (from the XML Schema
namespace for instances) to true:
<shipDate xsi:nil="true"></shipDate>
The nil
attribute is defined as part of the XML Schema namespace
for instances,
http://www.w3.org/2001/XMLSchema-instance,
and so it must appear in the instance document with a
prefix (such as xsi:) associated with that namespace.
(As with the xsd: prefix, the
xsi: prefix is used by convention only.) Note that
the nil mechanism applies only to element values, and not
to attribute values. An element with xsi:nil="true" may not
have any element content but it may still carry attributes.
3 Advanced Concepts I: Namespaces, Schemas & Qualification
A schema can be viewed as a
collection (vocabulary) of type definitions and element
declarations whose names belong to a particular namespace
called a target namespace. Target namespaces enable us
to distinguish between definitions and declarations from
different vocabularies. For example, target namespaces
would enable us to distinguish between the declaration for
element in the
XML Schema language vocabulary, and a declaration for
element in a hypothetical chemistry language
vocabulary. The former is part of the
http://www.w3.org/2001/XMLSchema target
namespace, and the latter is part of another target
namespace.
When we want to check that an
instance document conforms to one or more schemas (through
a process called schema validation), we need to identify
which element and attribute declarations and type
definitions in the schemas should be used to check which
elements and attributes in the instance document. The
target namespace plays an important role in the
identification process. We examine the role of the target
namespace in the next section.
The schema author also has several options that affect how
the identities of elements and attributes are represented
in instance documents. More specifically, the author can
decide whether or not the appearance of locally declared
elements and attributes in an instance must be qualified by
a namespace, using either an explicit prefix or implicitly
by default. The schema author's choice regarding
qualification of local elements and attributes has a number
of implications regarding the structures of schemas and
instance documents, and we examine some of these
implications in the following sections.
3.1 Target Namespaces & Unqualified Locals
In a new version of the purchase order schema, po1.xsd, we explicitly declare
a target namespace, and specify that both locally defined
elements and locally defined attributes must be
unqualified. The target namespace in po1.xsd is
http://www.example.com/PO1, as indicated by the
value of the
targetNamespace attribute.
Qualification of local elements and attributes can be
globally specified by a pair of attributes,
elementFormDefault and
attributeFormDefault, on the schema element, or can be
specified separately for each local declaration using the
form attribute.
All such attributes' values may each be set to
unqualified or qualified, to indicate
whether or not locally declared elements and attributes
must be unqualified.
In po1.xsd we globally
specify the qualification of elements and attributes by
setting the values of both
elementFormDefault and
attributeFormDefault to
unqualified. Strictly speaking, these settings are
unnecessary because the values are the defaults for the two
attributes; we make them here to highlight the contrast between
this case and other cases we describe later.
Purchase Order Schema with Target Namespace, po1.xsd
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="unqualified"
attributeFormDefault="unqualified">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="comment" type="string"/>
<complexType name="PurchaseOrderType">
<sequence>
<element name="shipTo" type="po:USAddress"/>
<element name="billTo" type="po:USAddress"/>
<element ref="po:comment" minOccurs="0"/>
<!-- etc. -->
</sequence>
<!-- etc. -->
</complexType>
<complexType name="USAddress">
<sequence>
<element name="name" type="string"/>
<element name="street" type="string"/>
<!-- etc. -->
</sequence>
</complexType>
<!-- etc. -->
</schema>
To see how the target namespace of this schema is
populated, we examine in turn each of the type
definitions and element declarations. Starting from the end
of the schema, we first define a type called
USAddress that consists of the elements
name, street, etc. One consequence of
this type definition is that the USAddress
type is included in the schema's target namespace. We next
define a type called PurchaseOrderType that
consists of the elements shipTo,
billTo, comment, etc.
PurchaseOrderType is also included in the schema's
target namespace. Notice that the type references in the
three element declarations are prefixed, i.e.
po:USAddress, po:USAddress and
po:comment, and the prefix is associated with the
namespace http://www.example.com/PO1. This is
the same namespace as the schema's target namespace, and so
a processor of this schema will know to look within this
schema for the definition of the type
USAddress and the declaration of the element
comment. It is also possible to refer to types in
another schema with a different target namespace, hence
enabling re-use of definitions and declarations between
schemas.
At the beginning of the schema
po1.xsd, we declare the elements
purchaseOrder and comment. They are
included in the schema's target namespace. The
purchaseOrder element's type is prefixed, for the
same reason that USAddress is prefixed. In
contrast, the comment element's type, string, is not prefixed. The
po1.xsd schema
contains a default namespace declaration, and so unprefixed
types such as string
and unprefixed elements such as element and complexType are
associated with the default namespace
http://www.w3.org/2001/XMLSchema. In fact, this
is the target namespace of XML Schema itself, and so a
processor of po1.xsd
will know to look within the schema of XML Schema
-- otherwise known as the "schema for schemas" -- for the
definition of the type
string and the declaration of the element called
element.
Let us now examine how the target namespace of the schema
affects a conforming instance document:
A Purchase Order with Unqualified Locals, po1.xml
<?xml version="1.0"?>
<apo:purchaseOrder xmlns:apo="http://www.example.com/PO1"
orderDate="1999-10-20">
<shipTo country="US">
<name>Alice Smith</name>
<street>123 Maple Street</street>
<!-- etc. -->
</shipTo>
<billTo country="US">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<!-- etc. -->
</billTo>
<apo:comment>Hurry, my lawn is going wild<!/apo:comment>
<!-- etc. -->
</apo:purchaseOrder>
The instance document declares one namespace,
http://www.example.com/PO1, and associates it with
the prefix apo:. This prefix is used to
qualify two elements in the document, namely
purchaseOrder and comment. The
namespace is the same as the target namespace of the schema
in po1.xsd, and so a
processor of the instance document will know to look in
that schema for the declarations of
purchaseOrder and comment. In fact,
target namespaces are so named because of the sense in
which there exists a target namespace for the elements
purchaseOrder and comment. Target
namespaces in the schema therefore control the validation
of corresponding namespaces in the instance.
The prefix apo: is applied to the global
elements purchaseOrder and
comment elements. Furthermore,
elementFormDefault and