Copyright ©2000 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
XML Schema Part 0: Primer is a non-normative document intended to provide an easily readable description of the XML Schema facilities and is oriented towards quickly understanding how to create schemas using the XML Schema language. XML Schema Part 1: Structures and XML Schema Part 2: Datatypes provide the complete normative description of the XML Schema definition language, and the primer describes the language features through numerous examples which are complemented by extensive references to the normative texts.
The XML Schema Part 0: Primer is a part of the W3C XML Activity.
This is a public working draft of XML Schema 1.0 for review by the public and by members of the World Wide Web Consortium. The XML Schema Working Group has agreed to its publication. Note that some sections of this draft may not be up-to-date with the XML Schema language described in Parts 1 and 2 of the XML Schema specification. Known discrepancies are noted in the text.
The Working Group does not anticipate further substantial changes to the syntax described here, although this is still a working draft, and is subject to change based on experience and on comment by the public, and other W3C working groups.
A list of current W3C working drafts can be found at http://www.w3.org/TR/. They may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress".
2 Basic Concepts: The Purchase Order
2.1 The Purchase Order Schema
2.2 Complex Type Definitions,
Element & Attribute Declarations
2.3 Simple Types
2.4 Anonymous Type
Definitions
2.5 Element Content
2.5.1 Complex Types from
Simple Types
2.5.2 Empty Content
2.5.3 Mixed Content
2.5.4 Default Content
2.6 Annotations
2.7 Building Content Models
2.8 Attribute Groups
2.9 Null Values
3. Advanced Concepts I: Namespaces, Schemas &
Qualification
3.1 Target Namespaces & Unqualified Locals
3.2 Qualified Locals
3.3 Global vs. Local Declarations
3.4 Undeclared Target Namespaces
4. Advanced Concepts II: The International
Purchase Order
4.1 A Schema in Multiple
Documents
4.2 Deriving Types by
Extension
4.3 Using Derived Types in
Instance Documents
4.4 Deriving Complex Types by
Restriction
4.5 Equivalence Classes
4.6 Abstract Elements and
Types
4.7 Preventing the Creation and
Use of Derived Types
5. Advanced Concepts III: The
Quarterly Report
5.1 Specifying
Uniqueness
5.2 Defining Keys
and their References
5.3 XML Schema
Constraints vs. XML 1.0 ID Attributes
5.4 Importing Types
5.5 Any Element, Any Attribute
5.6 schemaLocation
5.7 Conformance
A. Acknowledgements
B. Simple Types & Their
Facets
C. Regular Expressions
D. Index
E. Document History
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 XML-Namespaces. Each major section of the primer introduces new features of the language, and describes the features in the context of concrete examples.
Section 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 null values.
Section 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.
Section 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.
Section 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 an associated 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 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.
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 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 consists of a main element,
purchaseOrder, and the subelements
shipTo, billTo, and items.
These subelements in turn contain other subelements, and so
on, until a subelement such as price 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.
The purchase order schema is contained in the file
po.xsd:
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/1999/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.
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 the appearance in document instances of elements or attributes with specific names and types (both simple and complex). 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 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, Address is defined as a complex type, and
within the definition of Address we see five
element declarations and one attribute declaration:
| Defining the Address Type |
<xsd:complexType name="Address" >
<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:attribute name="country" type="xsd:NMTOKEN"
use="fixed" value="US"/>
</xsd:complexType>
|
The consequence of this definition is that any element
appearing in an instance whose type is declared to be
Address (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.
The first four of these elements will
each contain a string, and the fifth will contain a decimal
number. The element whose type is declared to be
Address may appear with an attribute called
country which must contain the string US.
The Address definition contains only
declarations involving simple types: string,
decimal and NMTOKEN. In contrast,
the purchaseOrderType definition contains
element declarations involving complex types, e.g.
Address, 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:element name="shipTo" type="Address" />
<xsd:element name="billTo" type="Address" />
<xsd:element ref="comment" minOccurs="0" />
<xsd:element name="items" type="Items" />
<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 Address. The
consequence of this definition is that any element
appearing in an instance (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 Address. The shipTo
and billTo elements may also carry the
country attribute that was declared as part of
Address.
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.
Both elements and attributes may be declared globally.
comment is one example of a global element
which we reference from an element declaration contained in
the PurchaseOrderType definition. We could
similarly declare attributes under schema, and reference
them using the ref attribute from attribute declarations
contained in type definitions.
The comment element is optional within
PurchaseOrderType because the value of the
minOccurs attribute in its declaration is 0. An
element is required to appear when the value of
minOccurs is 1. The maximum number of times an
element may appear is determined by the value of a
maxOccurs attribute in its declaration. This may be
a positive integer value such as 41, or the term
unbounded to indicate there is no maximum number of
occurrences. The default value for minOccurs is
1, but there is no default value for maxOccurs
per se: When an element is declared without a
maxOccurs attribute, the maximum number of the
element's occurrences is equal to the value of the
minOccurs attribute. If this value is also omitted,
the element must appear exactly once.
Attributes may appear once or not at all (the default), and so the
syntax for specifying occurrences of attributes is different than the
syntax for elements. In particular, a use attribute is
used in an attribute declaration to indicate whether the attribute is
required or optional, and if
optional whether the attribute's value is fixed
or whether there is a default. A second attribute,
value, provides any value that is called for. To illustrate,
po.xsd contains a declaration for the country
attribute, which is declared with use and value
values of fixed and US respectively. This
declaration means that the appearance of a country
attribute is optional, although its value must be US if
it does appear, and if it does not appear, a schema processor will
create a country attribute with this value.
The values of the attributes used in element and attribute declarations to constrain the occurrences of elements and attributes are summarised in Table 1.
| Table 1. Occurrence Constraints for Elements and Attributes | ||
|---|---|---|
|
Elements (minOccurs, maxOccurs) fixed, default |
Attributes use, value |
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, - | fixed, 37 | element/attribute may appear once, if it does appear its value must be 37 |
| (0, 1) -, 37 | default, 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 it does not appear 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 |
So far, we have described how to define new complex types
(e.g. PurchaseOrderType), and declare elements
(e.g. purchaseOrder) and attributes (e.g.
orderDate). These activities generally involve
naming, and the question naturally arises: What happens if
two things are given 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 is there to be
a conflict.
Here are some examples to illustrate when same names cause problems. If the two things are both types, say I define a complex type called US-States and a simple type called US-States, there is a conflict. If the two things are a type and an element or attribute, say I define a complex type called Address and I declare an element called Address, there is no conflict. If the two things are elements within different types (i.e. not global elements), say I declare one element called name as part of the Address 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'll explore the use of schema and namespaces in a later section.
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 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 an
example of each type.
| Table 2. Simple Types Built-In to XML Schema | ||
|---|---|---|
| Simple Type | Example(s) | Notes |
| string | Confirm this is electric | |
| boolean | true, false, 1, 0 | |
| float | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to single-precision 32-bit floating point, NaN is "not a number" |
| double | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to double-precision 64-bit floating point |
| decimal | -1.23, 0, 123.4, 1000.00 | |
| timeInstant | 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 |
| timePeriod | 1999-05-31T13:20 | |
| month | 1999-05 | May 1999 |
| year | 1999 | 1999 |
| century | 19 | the 1900's |
| recurringDate | --05-31 | every May 31st |
| recurringDay | ----31 | every 31st day |
| timeDuration | P1Y2M3DT10H30M12.3S | 1 year, 2 months, 3 days, 10 hours, 30 minutes, 12.3 seconds |
| recurringDuration | --05-31T13:20:00 | May 31st every year at 1.20pm Co-Ordinated Universal Time, format similar to timeInstant |
| binary | 100010 | |
| uriReference | http://www.example.com/, http://www.example.com/doc.html#ID5 | |
| ID | XML 1.0 ID attribute type | |
| IDREF | XML 1.0 IDREF attribute type | |
| ENTITY | XML 1.0 ENTITY attribute type | |
| NOTATION | XML 1.0 NOTATION attribute type | |
| language | en-GB, en-US, fr | valid values for xml:lang as defined in XML 1.0 |
| IDREFS | XML 1.0 IDREFS attribute type | |
| ENTITIES | XML 1.0 ENTITIES attribute type | |
| NMTOKEN | US | XML 1.0 NMTOKEN attribute type |
| NMTOKENS | US UK | XML 1.0 NMTOKENS attribute type |
| Name | shipTo | XML 1.0 Name type |
| QName | po:Address | XML Namespace QName |
| NCName | Address | XML Namespace NCName, i.e. a QName without the prefix and colon |
| integer | -126789, -1, 0, 1, 126789 | |
| nonPositiveInteger | -126789, -1, 0 | |
| negativeInteger | -126789, -1 | |
| long | -1, 12678967543233 | |
| int | -1, 126789675 | |
| short | -1, 12678 | |
| byte | -1, 126 | |
| nonNegativeInteger | 0, 1, 126789 | |
| unsignedLong | 0, 12678967543233 | |
| unsignedInt | 0, 1267896754 | |
| unsignedShort | 0, 12678 | |
| unsignedByte | 0, 126 | |
| positiveInteger | 1, 126789 | |
| date | 1999-05-31, ---05 | 5th day of every month |
| time | 13:20:00.000, 13:20:00.000-05:00 | |
| Note that 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. | ||
New simple types are defined by derivation from existing
simple types (built-in's and derived) through a technique
called restriction. A new type must have a name different
from the existing type, and the new type may constrain the
legal range of values obtained from the existing type. We
use the
simpleType element to define a new simple
type, and we can constrain its values by applying one or
more "facets". A complete listing 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 1
and 99 (inclusive). We base our definition on the built-in
simple type integer,
whose range of values also includes integers less than 1 and greater
than 99.
To define myInteger,
we limit the range of the integer base type by employing
two facets, minInclusive
and maxInclusive:
| Defining myInteger, Range 1-99 |
<xsd:simpleType name="MyInteger" base="xsd:integer"> <xsd:minInclusive value="1"/> <xsd:maxInclusive value="99"/> </xsd:simpleType> |
The example shows one particular combination of a base
type and a facet used to define myInteger, but
a look at the list of built-in
simple types and their facets 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 (shorthand for a product number)
is derived 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 letters":
| Defining the Simple Type "Sku" |
<xsd:simpleType name="Sku" base="xsd:string">
<xsd:pattern value="\d{3}-[A-Z]{2}"/>
</xsd:simpleType>
|
This regular expression language is described more fully in Appendix C.
XML Schema defines fourteen facets which are listed in
full in Appendix B. Among
these, the enumeration facet is one the most 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 US-State, derived from
string, whose value must be one of the standard
US state abbreviations:
| Using the Enumeration Facet |
<xsd:simpleType name="US-State" base="xsd:string"> <xsd:enumeration value="AK"/> <xsd:enumeration value="AL"/> <xsd:enumeration value="AR"/> <!-- and so on ... --> </xsd:simpleType> |
US-State 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.
The majority of simple types described in Table 2 are so-called atomic types,
for example, decimal and NMTOKEN.
The values of atomic types are indivisible from XML
Schema's point of view. In contrast, XML Schema has three
built-in list types that are comprised of sequences of
atomic types. 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". The three built-in lists types 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 complex types).
For example, to create a list of myInteger's:
<xsd:simpleType name='ListOfMyIntType' base='myInteger' derivedBy='xsd:list'/>
And an element in an instance document whose content
conforms to ListOfMyIntType is:
<listOfMyInt>47 25 99 3 25 1</listOfMyInt>
Several facets can be applied in the derivation of a new
list type: length, minLength,
maxLength, and enumeration. For
example, to create a list of exactly six US states, we can
derive a new list type from the US-State base
type, defined above, and constrain the number of items to
six:
| List Type for Six US States |
<xsd:simpleType name="SixUS-States" base="US-State" derivedBy="xsd:list"> <xsd:length value="6"/> </xsd:simpleType> |
Elements declared as having this type must have six items,
and each of the six items is one of the (atomic) values of
the enumerated type US-State, 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 fixed length list types whose base type is
string. For example, suppose a list type is defined
with a length facet equal to 3, and base type
string, then the following 3 item list is
legal:
Asia Europe Africa
But the following 3 "item" list is illegal:
Asia Europe South America
Even though "South America" may exist as a single string outside of the list, when it is included in the list, the whitespace between South and America effectively creates a fourth item, and so the latter example will not conform to the 3-item list type.
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 the element (or
attribute) declaration, and the declaration containing an
un-named type definition:
| Two Anonymous Type Definitions |
<xsd:complexType name="Items">
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:simpleType>
</xsd:element>
<xsd:element name="price" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs='0'/>
<xsd:attribute name="partNum" type="Sku"/>
</xsd:complexType>
</xsd:element>
</xsd:complexType>
|
In the case of the item element, it has an
anonymous complex type consisting of the elements
productName, quantity,
price, comment, and
shipDate, and an attribute called
partNum. In the case of the quantity
element, it has an anonymous simple type derived from
integer whose value ranges between 1 and 99.
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. price). 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.
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='EU'>423.46</internationalPrice>
The purchase order schema declares a price
element that is a starting point:
<xsd:element name="price" 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 base='xsd:decimal' derivedBy='extension'>
<xsd:attribute name='currency' type='xsd:string' />
</xsd:complexType>
</xsd:element>
|
We use the complexType element to define the
new (anonymous) type, and we refer to decimal
in the base attribute to indicate it is the
simple type from which we are deriving the new type. We add
a currency attribute using a standard
attribute declaration, and because we want to add
this attribute to the simple type, we must signal our
intent by stating derivedBy='extension'. (We
cover type derivation in detail in Section
4). The price element declared in this way
will appear in an instance as shown in the example above.
Now suppose that we want the price 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='EU' value='423.46' />
Such an element has no content at all, we say that its
content model is empty:
| An Empty Complex Type |
<xsd:element name='internationalPrice'>
<xsd:complexType content='empty'>
<xsd:attribute name='currency' type='xsd:string' />
<xsd:attribute name='value' type='xsd:decimal' />
</xsd:complexType>
</xsd:element>
|
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. The latter
style of construction is enabled through the mixed value
of the content attribute.
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 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 content='mixed'>
<xsd:element name='salutation'>
<xsd:complexType content='mixed'>
<xsd:element name='name' type='xsd:string'/>
</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:complexType>
</xsd:element>
|
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 sum, XML Schema
provides full schema validation of mixed models in contrast to the partial
schema validation provided by XML 1.0.
In previous sections, we have defined new complex types
without reference to the content attribute, and so it is
reasonable to ask what content model was used in those
definitions. The default content model for a complex type
is called elementOnly, i.e. the complex type
may contain elements and attributes. In general, the
content acceptable by mixed and
elementOnly models is the same, except
mixed models also accept character data appearing
before, after and between elements.
elementOnly is the content model that applies
when we derive complex types from other complex types, but
when we derive a complex type from a simple type (as we did
in Section 2.5.1), the
content model is called textOnly. In fact, we
can define a complex type in terms of
textOnly:
| A textOnly Complex Type |
<xsd:element name='internationalPrice'>
<xsd:complexType content='textOnly'>
<xsd:attribute name='currency' type='xsd:string' />
</xsd:complexType>
</xsd:element>
|
The content of the anonymous type defined in this way is
unconstrained, so the
element value may be 423.46, but
legitimately it may be any other sequence of characters as
well. In general it is probably better to avoid such
unconstrained type definitions in favour of constrained
type definitions such as decimal and string.
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.
The 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 is one of
the
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
elements, which was used by an application to automatically
generate text for the XML Schema Part 2 document.
Both documentation and 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'>
<annotation>
<documentation>element declared with anonymous type</documentation>
</annotation>
<xsd:complexType content='empty'>
<annotation>
<documentation>empty anonymous type with 2 attributes</documentation>
</annotation>
<xsd:attribute name='currency' type='xsd:string' />
<xsd:attribute name='value' type='xsd:decimal' />
</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.
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 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.
Note that the constraints do not apply to attributes. These
constraints mirror those available in XML 1.0 plus some
additional constraints.
XML Schema enables a group 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 modify the PurchaseOrderType definition from the purchase order schema using two groups so that purchase orders may contain either separate shipping and billing addresses, or a single address for those cases in which the shipper and biller are co-located:
| Nested Choice and Sequence Groups |
<xsd:complexType name="PurchaseOrderType">
<xsd:choice>
<xsd:group ref="shipAndBill" />
<xsd:element name="singleAddress" type="Address" />
</xsd:choice>
<xsd:element ref="comment" minOccurs="0" />
<xsd:element name="items" type="Items" />
<xsd:attribute name="orderDate" type="xsd:date" />
</xsd:complexType>
<xsd:group name="shipAndBill">
<xsd:sequence>
<xsd:element name="shipTo" type="Address" />
<xsd:element name="billTo" type="Address" />
</xsd:sequence>
</xsd:group>
|
A 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 singleAddress. Hence, in an
instance document, the purchaseOrder element
must contain either a singleAddress element or
a shipTo element followed by a
billTo element. Note that the sequence
element used in the definition of shipAndBill
is not strictly necessary because the content model of a
named group is a sequence by default.
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
any element in the content model may appear no 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="Address"/>
<xsd:element name="billTo" type="Address"/>
<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:all>
<xsd:element name="shipTo" type="Address"/>
<xsd:element name="billTo" type="Address"/>
<xsd:element name="items" type="Items" />
</xsd:all>
<xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/>
<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.
Suppose we want to provide more information about the
items in a purchase order, by adding attributes to the
item element indicating whether or not the
item is in stock, weight, and preferred shipping method.
One way to add these attributes is to add more attribute
declarations to the Item's type definition:
| Adding Attributes to the Inline Type Definition |
<xsd:element name="Item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:simpleType>
</xsd:element>
<xsd:element name="price" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs='0'/>
<xsd:attribute name="partNum" type="Sku"/>
<xsd:attribute name="weight" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType base="string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:simpleType>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
|
Alternatively, we can create a named attribute group
containing these attributes 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:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:simpleType>
</xsd:element>
<xsd:element name="price" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs='0'/>
<xsd:attributeGroup ref="ItemDelivery"/>
</xsd:complexType>
</xsd:element>
<xsd:attributeGroup name="ItemDelivery">
<xsd:attribute name="partNum" type="Sku"/>
<xsd:attribute name="weight" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType base="xsd:string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:simpleType>
</xsd:attribute>
</xsd:attributeGroup>
|
Using an attribute group in this way can improve the readability of schema, and facilitates updating schema 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 both attribute declarations and attribute group references must appear at the end of complex type definitions.
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 null mechanism
which enables an element to appear with or without a non-null value.
XML Schema's null mechanism
involves an "out of band" null signal. In other words,
there is no actual null value that appears as element
content, instead there is an attribute to indicate that the
element content is null. To illustrate, we can modify the
shipDate element declaration so that nulls can
be signalled:
<xsd:element name="shipDate" type="xsd:date" nullable="true"/>
And to explictly represent that shipDate has
a null value in the instance document, we set the null
attribute (from the XML Schema namespace for instances) to
true:
<shipDate xsi:null="true"></shipDate>
The null attribute is defined as part of the XML Schema
namespace for instances
(http://www.w3.org/1999/XMLSchema-instance),
and so it must appear in the instance document with a
prefix (xsi:) associated with that namespace. (As with
the xsd: prefix, the xsi: prefix is used
by convention only).
Note that the null mechanism applies only to element
values, and not to attribute values. An element with
xsi:null="true" may not have any element content but
it may still carry attributes.
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.
The target namespace enables 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/1999/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.
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 qualified.
Strictly speaking, this is unnecessary because these are the default
values of the two attributes, but we do so to highlight the contrast
between this case and others we describe in subsequent sections.
To see how the target namespace of this schema is
populated, we'll examine in turn each of the type
definitions and element declarations. Starting from the end
of the schema, we first define a type called
Address that consists of the elements
name, street, etc. One consequence of
this type definition is that the Address 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:Address, po:Address 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 Address
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 Address 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/1999/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:
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 attributeFormDefault
require that the prefix is not applied to any of the the locally declared
elements such as shipTo, billTo,
name and street, and it is not applied to
any of the attributes (which were all declared locally). The
purchaseOrder and
comment are global elements because they are
declared in the context of the schema as a whole rather
than within the context of a particular type. For example,
the declaration of purchaseOrder appears as a
child of the schema element in
po1.xsd, whereas the declaration of
shipTo appears as a child of the
complexType element that defines
Address.
When local elements and attributes are not required to be qualified, an
instance author may require more or less knowledge about the details of
the schema to create schema valid instance documents. More specifically,
if the author can be sure that only the root element
(such as purchaseOrder) is global, then it is a simple
matter to qualify only the root element. Alternatively, the author
may know that all the elements are declared globally, and so all the
elements in the instance document can be prefixed, perhaps taking advantage
of a default namespace declaration. (We examine this approach in
Section 3.3).
On the other hand, if there is no uniform pattern of global and local
declarations, the author will need detailed knowledge of the
schema to correctly prefix global elements (and attributes).
Elements and attributes can be independently required
to be qualified, although we'll start by describing qualification of local
elements. To specify that all locally declared elements in a schema must
be qualified, we set the value of
elementFormDefault
to qualified:
| Modifications to po1.xsd for Qualified Locals |
<schema xmlns="http://www.w3.org/1999/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="comment" type="string"/>
<complexType name="PurchaseOrderType">
<!-- etc -->
</complexType>
<!-- etc -->
</schema>
|
And in this conforming instance document, we qualify all the elements explicitly:
| A Purchase Order with Explicitly Qualified Locals |
<?xml version="1.0"?>
<apo:purchaseOrder xmlns:apo="http://www.example.com/PO1"
orderDate="1999-10-20">
<apo:shipTo country="US">
<apo:name>Alice Smith</apo:name>
<apo:street>123 Maple Street</apo:street>
<!-- etc -->
</apo:shipTo>
<apo:billTo country="US">
<apo:name>Robert Smith</apo:name>
<apo:street>8 Oak Avenue</apo:street>
<!-- etc -->
</apo:billTo>
<apo:comment>Hurry, my lawn is going wild!</apo:comment>
<!-- etc -->
</apo:purchaseOrder>
|
Alternatively, we can replace the explicit qualification of every element with implicit qualification provided by a default namespace, as shown here in po2.xml:
In po2.xml, all the elements in the instance belong to the same namespace, and the namespace statement declares a default namespace that applies to all the elements in the instance. Hence, it is unnecessary to explicitly prefix any of the elements. As another illustration of using qualified elements, the schemas in Section 5 all require qualified elements.
Qualification of attributes is very similar to the qualification of
elements. Attributes that must be qualified, either because they are
declared globally or because the
attributeFormDefault
attribute is set to qualified, appear prefixed in instance
documents. One example of a qualified attribute is the
xsi:null attribute that was
introduced in Section 2.9. In fact, attributes
that are required to be qualified must be explicitly prefixed because the
XML-Namespaces
specification does not provide a mechanism for defaulting the namespaces
of attributes. Attributes that are not required to be qualified
appear in instance documents without prefixes, which is the
typical case.
The qualification mechanism we have described so far has
controlled all local element and attribute declarations within a particular
target namespace. It is also possible to control qualification on a
declaration by declaration basis using the
form attribute. For example,
to require that the locally declared attribute publicKey
is qualified in instances, we declare it in the following way:
| Requiring Qualification of Single Attribute |
<schema xmlns="http://www.w3.org/1999/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- etc -->
<element name="secure">
<complexType>
<!-- element declarations -->
<attribute name="publicKey" type="binary" form="qualified">
</complexType>
</element>
</schema>
|
Notice that the value of
the form attribute overides
the value of the
attributeFormDefault
attribute for the publicKey attribute only. Also,
the form attribute can be applied
to an element declaration in the same manner. An instance document that
conforms to the schema is:
| Instance with a Qualified Attribute |
<?xml version="1.0"?>
<purchaseOrder xmlns="http://www.example.com/PO1"
xmlns:po="http://www.example.com/PO1"
orderDate="1999-10-20">
<!-- etc -->
<secure po:publicKey="11110000111110100010">
<!-- etc -->
</secure>
</purchaseOrder>
|
Another authoring style, when all the element names are unique within
a namespace, is to create a schema in which all elements are global.
This is similar in effect to the use of <!ELEMENT> in a DTD.
In the example below, we have modified po1.xsd
such that all the elements are declared globally. Notice that we have
omitted the
elementFormDefault
and attributeFormDefault
attributes in this example to emphasise that their values are irrelevant
when there are only global element and attribute declarations.
| Modified version of po1.xsd using only global element declarations |
<schema xmlns="http://www.w3.org/1999/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="shipTo" type="po:Address"/>
<element name="billTo" type="po:Address"/>
<element name="comment" type="string"/>
<element name="name" type="string"/>
<element name="street" type="string"/>
<complexType name="PurchaseOrderType">
<element ref="po:shipTo"/>
<element ref="po:billTo"/>
<element ref="po:comment" minOccurs="0"/>
<!-- etc -->
</complexType>
<complexType name="Address">
<element ref="po:name"/>
<element ref="po:street"/>
<!-- etc -->
</complexType>
<!-- etc -->
</schema>
|
This "global" version of po1.xsd will validate the instance document po2.xml which, as we described previously, is also schema valid against the "qualified" version of po1.xsd. In other words, both schema approaches can validate the same, namespace defaulted, document. Thus, in one respect the two schema approaches are similar, although in another important respect the two schema approaches are very different. Specifically, when all elements are declared globally, it is not possible to take advantage of local names. For example, you can only declare one global element called "title". However, you can locally declare one element called "title" that has a string type, and is a subelement of "book"; And within the same schema (target namespace) you can declare a second element also called "title" that is an enumeration of the values "Mr Mrs Ms".
In Section 2 we explained the basics of XML Schema using a schema that did not declare a target namespace and an instance document that did not declare a namespace. So the question naturally arises: What is the target namespace in these examples and how is it referenced?
In the purchase order schema, po.xsd, we did
not declare a target namespace for the schema, nor did we
declare a prefix (like po: above) associated
with the schema's target namespace with which we could
refer to types and elements defined and declared within the
schema. The consequence of not declaring a target namespace
in a schema is that the definitions and declarations from
that schema, such as Address and
purchaseOrder, are referenced without namespace
qualification. In other words there is no explicit
namespace prefix applied to the references nor is there any
implicit namespace applied to the reference by default. So
for example, the purchaseOrder element is
declared using the type reference
PurchaseOrderType. In contrast, all the XML Schema
elements and types used in po.xsd are
explicitly qualified with the prefix xsd: that
is associated with the XML Schema namespace.
Element declarations from a schema with no target
namespace validate unqualified elements in the instance
document. That is, they validate elements for which no
namespace qualification is provided by either an explicit
prefix or by default (xmlns:). So, to validate
a traditional XML 1.0 document which does not use
namespaces at all, you must provide a schema with no target
namespace. Of course, there are many XML 1.0 documents that
do not use namespaces, so there will be many schema
documents written without target namespaces; you must be
sure to give to your processor a schema document that
corresponds to the vocabulary you wish to validate.
The purchase order schema described in Chapter 2 was contained in a single document, and most of the schema constructions-- such as element declarations and type definitions-- were constructed from scratch. In reality, schema authors will want to compose schemas from constructions located in multiple documents, and create new types based on existing types. In this section, we examine mechanisms that enable such compositions and creations.
As schemas become larger, it is often desirable to divide
their content among several schema documents for purposes
such as ease of maintenance, access control, and
readability. For these reasons, we have taken the schema
constructs concerning addresses out of po.xsd,
and put them in a new file called address.xsd.
The modified purchase order schema file is called
ipo.xsd:
The file containing the address constructs is:
The various purchase order and address constructions are
now contained in two schema files, ipo.xsd and
address.xsd. To include these constructions as
part of the international purchase order schema, in other
words to include them in the international purchase order's
namespace, ipo.xsd contains the
include element:
<include schemaLocation="http://www.example.com/schemas/address.xsd"/>
The effect of this include element is to bring in
the definitions and declarations contained in address.xsd,
and make them available as part of the international purchase order
schema target namespace.
The one important caveat to using include is
that the target namespace of the included constructions
must be the same as the target namespace of the including
schema, in this case
http://www.example.com/IPO.
In this example, we have shown only one including document and one included document. In practice it is possible to include more than one document using multiple include elements, and documents can include documents that themselves include other documents. However, nesting is legal only if all the included parts of the schema are declared with the same target namespace.
Instance documents that conform to schema whose
definitions span multiple schema documents need only
reference the 'topmost' document, and the common namespace,
and it is the responsibility of the processor to gather
together all the definitions specified in the various
included documents. So in our example, the instance
document ipo.xml (see Section 4.3) references only the
common target namespace,
http://www.example.com/IPO, and the one schema file
http://www.example.com/schemas/ipo.xsd. The
processor is responsible for obtaining the schema file
address.xsd.
In Section 5.4 we describe how schemas can be used to validate content from more than one namespace.
To create our address constructs, we start by creating a
complex type called Address in the usual way
(see address.xsd). The Address
type contains the basic elements of an address: a name, a
street and a city. From this starting point we derive two
new complex types that contain all the elements of the
original type plus additional elements that are specific to
addresses in the US and the UK. The technique we use here to
derive new (complex) address types by extending an existing type is the
same technique we used in
in Section 2.5.1, except that
our base type here is a complex type whereas our base type in the
previous section was a simple type.
We create the two new complex types,
US-Address and UK-Address, using the
complexType element along with values for
base and derivedBy attributes.
When a complex type is derived by extension, its effective
content model is the content model of the base type plus
the content model specified in the type derivation. Furthermore,
the two content models are treated as two children of a
sequential group.
In the case of UK-Address, the
content model of UK-Address is the content
model of Address plus the declarations for a postcode element
and an export-code attribute. This is like defining the
UK-Address from scratch as follows:
| Example |
<complexType name="UK-Address">
<sequence>
<!-- content model of Address -->
<element name="name" type="string"/>
<element name="street" type="string"/>
<element name="city" type="string"/>
<!-- appended declarations -->
<element name="postcode" type="ipo:UK-Postcode"/>
<attribute name="export-code" type="positiveInteger"
use="fixed" value="1"/>
</sequence>
</complexType>
|
In our example scenario, purchase orders are generated in
response to customer orders which may involve shipping and
billing addresses in different countries. The international
purchase order, ipo.xml below, illustrates one
such case where goods are shipped to England and the bill
is sent to a US address. Clearly it is very useful if the
schema for international purchase orders does not have to
spell out every possible combination of international
addresses for billing and shipping, and even more so if we
can add new complex types of international address simply
by creating new derivations of Address.
XML Schema allows us to define the billTo and
shipTo elements as Address types
(see ipo.xsd) but to use instances of
international addresses in place of instances of
Address. In other words, an instance document whose
content conforms to the UK-Address type will
be valid if that content appears within the document at a
location where an Address is expected
(assuming the UK-Address content itself is
valid). To make this feature of XML Schema work, and to
identify exactly which derived type is intended, the
derived type must be identified in the instance document.
The type is identified using the xsi:type
attribute which is part of the XML Schema instance
namespace. In the example, ipo.xml, use of the
UK-Address and US-Address
derived types is identified through the values assigned to
the xsi:type attributes.
In Section 4.7 we'll see how to prevent derived types from being used in this sort of substitution.
In addition to deriving new complex types by extending content models, it is also possible to derive new types by restricting the content models of existing types. Restriction of complex types is conceptually the same as restriction of simple types, except that the restriction of complex types involves a type's declarations rather than the acceptable range of a simple type's values. A complex type derived by restriction is very similar to its base type, except that its declarations are more limited than the corresponding declarations in the base type. In fact, the values represented by the new type are a subset of the values represented by the base type (as is also the case with restriction of simple types). In other words, an application prepared for the values of the base type would not be surprised by the values of the restricted type.
For example, suppose we want to update our definition of
the list of items in an international purchase
order so that it must contain at least one
item on order; The schema shown in
ipo.xsd allows an items element to
appear without any child item elements. To