XML Schema Part 0: Primer Second Edition (original) (raw)

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. 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](#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, 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](#po.xsd):

The purchase order schema consists of a `` [schema](#element-schema) element and a variety of subelements, most notably [element](#element-element), [complexType](#element-complexType), and [simpleType](#element-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-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](#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](#element-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-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-element) element, and attributes are declared using the [attribute](#element-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:

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](#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](#string), [ decimal](#decimal) and [ NMTOKEN](#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-type) attribute to identify the type, regardless of whether the type is simple or complex.

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](#po.xml)) whose type is declared to be PurchaseOrderType must consist of elements namedshipTo and billTo, each containing the five subelements (name, street, city, state andzip) that were declared as part of USAddress. The shipTo and billTo elements may also carry the country attribute that was declared as part ofUSAddress.

The PurchaseOrderType definition contains anorderDate 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-ref) attribute must reference a global element, i.e. one that has been declared under[schema](#element-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](#string).

2.2.1 Occurrence Constraints

The comment element is optional within PurchaseOrderType because the value of the `` [ minOccurs](#attribute-minOccurs) attribute in its declaration is 0. In general, an element is required to appear when the value of[minOccurs](#attribute-minOccurs) is 1 or more. The maximum number of times an element may appear is determined by the value of a [maxOccurs](#attribute-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](#attribute-minOccurs) and the [maxOccurs](#attribute-maxOccurs) attributes is 1. Thus, when an element such as comment is declared without a [ maxOccurs](#attribute-maxOccurs) attribute, the element may not occur more than once. Be sure that if you specify a value for only the [ minOccurs](#attribute-minOccurs) attribute, it is less than or equal to the default value of [ maxOccurs](#attribute-maxOccurs), i.e. it is 0 or 1. Similarly, if you specify a value for only the [maxOccurs](#attribute-maxOccurs) attribute, it must be greater than or equal to the default value of[minOccurs](#attribute-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-use) attribute to indicate whether the attribute is required (see for example, the partNum attribute declaration in [ po.xsd](#po.xsd)), optional, or evenprohibited.

Default values of both attributes and elements are declared using thedefault 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-attr-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 ofoptional for [use](#attribute-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-attr-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](#po.xsd) contains a declaration for the country attribute, which is declared with a [fixed](#attribute-attr-fixed) value US. This declaration means that the appearance of a country attribute in an instance document is optional (the default value of [use](#attribute-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 bothfixed and default attributes.

The values of the attributes used in element and attribute declarations to constrain their occurrences are summarized inTable 1.

minOccurs, maxOccurs)

fixed, default

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, andmaxOccurs 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

n/a

optional, 37, -

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

n/a

optional, -, 37

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-schema) element. Once declared, a global element or a global attribute can be referenced in one or more declarations using the [ref](#attribute-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](#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](#po.xsd), can appear as the top-level element in[po.xml](#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](#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-ref) attribute, they must use the [type](#attribute-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](#string) and [decimal](#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](#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.

string

Confirm this is electric

normalizedString

Confirm this is electric

see (3)

token

Confirm this is electric

see (4)

base64Binary

GpM7

hexBinary

0FB7

integer

...-1, 0, 1, ...

see (2)

positiveInteger

1, 2, ...

see (2)

negativeInteger

... -2, -1

see (2)

nonNegativeInteger

0, 1, 2, ...

see (2)

nonPositiveInteger

... -2, -1, 0

see (2)

long

-9223372036854775808, ... -1, 0, 1, ... 9223372036854775807

see (2)

unsignedLong

0, 1, ... 18446744073709551615

see (2)

int

-2147483648, ... -1, 0, 1, ... 2147483647

see (2)

unsignedInt

0, 1, ...4294967295

see (2)

short

-32768, ... -1, 0, 1, ... 32767

see (2)

unsignedShort

0, 1, ... 65535

see (2)

byte

-128, ...-1, 0, 1, ... 127

see (2)

unsignedByte

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

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

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 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-simpleType) element to define and name the new simple type. We use the [restriction](#element-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](#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](#integer) base type by employing two facets called [minInclusive](#element-minInclusive) and [maxInclusive](#element-maxInclusive):

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](#string). Furthermore, we constrain the values of SKU using a facet called [pattern](#element-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":

This regular expression language is described more fully in Appendix D.

XML Schema defines twelve facets which are listed in Appendix B. Among these, the [enumeration](#element-enumeration) facet is particularly useful and it can be used to constrain the values of almost every simple type, except the [boolean](#boolean) type. The [enumeration](#element-enumeration) facet limits a simple type to a set of distinct values. For example, we can use the [ enumeration](#element-enumeration) facet to define a new simple type called USState, derived from [string](#string), whose value must be one of the standard US state abbreviations:

USState would be a good replacement for the`` [string](#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](#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](#NMTOKENS) is a list type, and an element of this type would be a white-space delimited list of [ NMTOKEN](#NMTOKEN)'s, such as "US UK FR". XML Schema has three built-in list types, they are [ NMTOKENS](#NMTOKENS), [ IDREFS](#IDREFS), and [ ENTITIES](#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:

And an element in an instance document whose content conforms to listOfMyIntType is:

Several facets can be applied to list types: `` [length](#element-length),[minLength](#element-minLength),[maxLength](#element-maxLength), [pattern](#element-pattern), and [ enumeration](#element-enumeration). For example, to define a list of exactly six US states (SixUSStates), we first define a new list type called USStateList fromUSState, and then we derive SixUSStates by restricting USStateList to only six items:

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:

Note that it is possible to derive a list type from the atomic type `` [string](#string). However, a [string](#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](#string). For example, suppose we have defined a list type with a [ length](#element-length) facet equal to 3, and base type [string](#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.

2.4 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](#attribute-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](#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](#attribute-type)= in an element (or attribute) declaration, and by the presence of an un-named (simple or complex) type definition:

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`` [positiveInteger](#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.

2.5.2 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:

Notice the text appearing between elements and their child elements. Specifically, text appears between the elementssalutation, quantity, productName and shipDate which are all children of letterBody, and text appears around the element name which is the child of a child ofletterBody. The following snippet of a schema declares letterBody:

The elements appearing in the customer letter are declared, and their types are defined using the `` [element](#element-element) and [complexType](#element-complexType) element constructions we have seen before. To enable character data to appear between the child-elements of letterBody, the [ mixed](#attribute-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.

2.5.4 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-documentation) element, which is the recommended location for human readable material. We recommend you use the xml:lang attribute with any[documentation](#element-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](#element-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](#element-appinfo) is aschema 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](#element-appinfo) elements, and this information was used by an application to automatically generate text for the XML Schema Part 2 document.

Both [documentation](#element-documentation) and[appinfo](#element-appinfo) appear as subelements of [ annotation](#element-annotation), which may itself appear at the beginning of most schema constructions. To illustrate, the following example shows [annotation](#element-annotation) elements appearing at the beginning of an element declaration and a complex type definition:

The `` [ annotation](#element-annotation) element may also appear at the beginning of other schema constructions such as those indicated by the elements [ schema](#element-schema), [ simpleType](#element-simpleType), and [attribute](#element-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](#attribute-minOccurs) (e.g. in comment), or be otherwise constrained depending upon the values of [minOccurs](#attribute-minOccurs) and [maxOccurs](#attribute-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:

The `` [choice](#element-choice) group element allows only one of its children to appear in an instance. One child is an inner [group](#element-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](#element-choice) group is followed by the comment and items element declarations, and both the [choice](#element-choice) group and the element declarations are children of a [sequence](#element-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](#element-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](#attribute-minOccurs) and [maxOccurs](#attribute-maxOccurs) are 0 and 1. For example, to allow the child elements of purchaseOrder to appear in any order, we could redefine PurchaseOrderType as:

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](#element-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](#element-all) group must appear as the sole child at the top of a content model. In other words, the following is illegal:

Finally, named and un-named groups that appear in content models (represented by `` [ group](#element-group) and [ choice](#element-choice), [ sequence](#element-sequence), [ all](#element-all) respectively) may carry [minOccurs](#attribute-minOccurs) and [maxOccurs](#attribute-maxOccurs) attributes. By combining and nesting the various groups provided by XML Schema, and by setting the values of [minOccurs](#attribute-minOccurs) and[maxOccurs](#attribute-maxOccurs), it is possible to represent any content model expressible with an XML 1.0 DTD. Furthermore, the [all](#element-all) group provides additional expressive power.

2.9 Nil Values

One of the purchase order items listed in `` [po.xml](#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 thatshipDate has a nil value in the instance document, we set the [nil](#attribute-xsinil) attribute (from the XML Schema namespace for instances) to true:

The [nil](#attribute-xsinil) 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](#attribute-xsinil)="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](#element-element) in the XML Schema language vocabulary, and a declaration forelement 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](#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](#po1.xsd) is http://www.example.com/PO1, as indicated by the value of the [ targetNamespace](#attribute-targetNamespace) attribute.

Qualification of local elements and attributes can be globally specified by a pair of attributes, [ elementFormDefault](#attribute-elementFormDefault) and [ attributeFormDefault](#attribute-attributeFormDefault), on the [schema](#element-schema) element, or can be specified separately for each local declaration using the[form](#attribute-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](#po1.xsd) we globally specify the qualification of elements and attributes by setting the values of both [ elementFormDefault](#attribute-elementFormDefault) and [ attributeFormDefault](#attribute-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.

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](#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](#string), is not prefixed. The [po1.xsd](#po1.xsd) schema contains a default namespace declaration, and so unprefixed types such as [string](#string) and unprefixed elements such as [element](#element-element) and [complexType](#element-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](#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](#string) and the declaration of the element called[element](#element-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](#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 elementspurchaseOrder 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](#attribute-elementFormDefault) and [ attributeFormDefault](#attribute-attributeFormDefault) require that the prefix is_not_ applied to any of 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-schema) element in [ po1.xsd](#po1.xsd), whereas the declaration of shipTo appears as a child of the [complexType](#element-complexType) element that defines PurchaseOrderType.

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 Global vs. Local Declarations (§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.

3.2 Qualified Locals

Elements and attributes can be independently required to be qualified, although we start by describing the qualification of local elements. To specify that all locally declared elements in a schema must be qualified, we set the value of `` [ elementFormDefault](#attribute-elementFormDefault) to qualified:

And in this conforming instance document, we qualify all the elements explicitly:

Alternatively, we can replace the explicit qualification of every element with implicit qualification provided by a default namespace, as shown here in `` [ po2.xml](#po2.xml):

In `` [po2.xml](#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 Advanced Concepts III: The Quarterly Report (§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-attributeFormDefault) attribute is set to qualified, appear prefixed in instance documents. One example of a qualified attribute is the [xsi:nil](#attribute-xsinil) attribute that was introduced in Nil Values (§2.9). In fact, attributes that are required to be qualified must be explicitly prefixed because the Namespaces in XML 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-form) attribute. For example, to require that the locally declared attribute publicKey is qualified in instances, we declare it in the following way:

Notice that the value of the `` [form](#attribute-form) attribute overrides the value of the [ attributeFormDefault](#attribute-attributeFormDefault) attribute for the publicKey attribute only. Also, the [form](#attribute-form) attribute can be applied to an element declaration in the same manner. An instance document that conforms to the schema is:

3.3 Global vs. Local Declarations

Another authoring style, applicable when all element names are unique within a namespace, is to create schemas 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 the original `` [po1.xsd](#po1.xsd) such that all the elements are declared globally. Notice that we have omitted the [ elementFormDefault](#attribute-elementFormDefault) and [ attributeFormDefault](#attribute-attributeFormDefault) attributes in this example to emphasize that their values are irrelevant when there are only global element and attribute declarations.

This "global" version of [po1.xsd](#po1.xsd) will validate the instance document [ po2.xml](#po2.xml) which, as we described previously, is also schema valid against the "qualified" version of [po1.xsd](#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". 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".

3.4 Undeclared Target Namespaces

In Basic Concepts: The Purchase Order (§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](#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 asUSAddress 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](#po.xsd) are explicitly qualified with the prefix xsd: that is associated with the XML Schema namespace.

In cases where a schema is designed without a target namespace, it is strongly recommended that all XML Schema elements and types are explicitly qualified with a prefix such as xsd: that is associated with the XML Schema namespace (as in [po.xsd](#po.xsd)). The rationale for this recommendation is that if XML Schema elements and types are associated with the XML Schema namespace by default, i.e. without prefixes, then references to XML Schema types may not be distinguishable from references to user-defined types.

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.

4 Advanced Concepts II: The International Purchase Order

The purchase order schema described in Basic Concepts: The Purchase Order (§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 to create new types based on existing types. In this section, we examine mechanisms that enable such compositions and creations.

4.1 A Schema in Multiple Documents

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](#po.xsd), and put them in a new file called [address.xsd](#address.xsd). The modified purchase order schema file is called [ipo.xsd](#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](#ipo.xsd) and[address.xsd](#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](#ipo.xsd) contains the [include](#element-include) element:

The effect of this [ include](#element-include) element is to bring in the definitions and declarations contained in [ address.xsd](#address.xsd), and make them available as part of the international purchase order schema target namespace. The one important caveat to using [include](#element-include) is that the target namespace of the included components must be the same as the target namespace of the including schema, in this case http://www.example.com/IPO. Bringing in definitions and declarations using the [ include](#element-include) mechanism effectively adds these components to the existing target namespace. In Redefining Types & Groups (§4.5), we describe a similar mechanism that enables you to modify certain components when they are brought in.

In our 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](#element-include) elements, and documents can include documents that themselves include other documents. However, nesting documents in this manner 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. In our example above, the instance document [ipo.xml](#ipo.xml) (seeUsing Derived Types in Instance Documents (§4.3)) references only the common target namespace, http://www.example.com/IPO, and (by implication) the one schema filehttp://www.example.com/schemas/ipo.xsd. The processor is responsible for obtaining the schema file[address.xsd](#address.xsd).

In Importing Types (§5.4) we describe how schemas can be used to validate content from more than one namespace.

4.2 Deriving Types by Extension

To create our address constructs, we start by creating a complex type called Address in the usual way (see `` [address.xsd](#address.xsd)). The Address type contains the basic elements of an address: a name, a street and a city. (Such a definition will not work for all countries, but it serves the purpose of our example.) 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 Complex Types from Simple Types (§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 define the two new complex types, USAddress and UKAddress, using the[complexType](#element-complexType) element. In addition, we indicate that the content models of the new types are complex, i.e. contain elements, by using the [ complexContent](#element-complexContent) element, and we indicate that we are extending the base type Address by the value of the [ base](#attribute-base) attribute on the [extension](#element-extension) element.

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 UKAddress, the content model of UKAddress is the content model of Address plus the declarations for a postcode element and an exportCode attribute. This is like defining the UKAddress from scratch as follows:

4.3 Using Derived Types in Instance Documents

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](#ipo.xml) below, illustrates one such case where goods are shipped to the UK and the bill is sent to a US address. Clearly it is better 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 thebillTo and shipTo elements asAddress types (see [ ipo.xsd](#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 UKAddress type will be valid if that content appears within the document at a location where anAddress is expected (assuming the UKAddress 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-xsitype) attribute which is part of the XML Schema instance namespace. In the example, [ipo.xml](#ipo.xml), use of the UKAddress and USAddress derived types is identified through the values assigned to the [xsi:type](#attribute-xsitype) attributes.

In Controlling the Creation & Use of Derived Types (§4.8) we describe how to prevent derived types from being used in this sort of substitution.

4.4 Deriving Complex Types by Restriction

In addition to deriving new complex types by extending content models, it is 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 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 a purchase order so that it must contain a comment; the schema shown in `` [ipo.xsd](#ipo.xsd) allows a purchaseOrder element to appear without any child comment elements. To create our new RestrictedPurchaseOrderType type, we define the new type in the usual way, indicate that it is derived by restriction from the base type PurchaseOrderType , and provide a new (more restrictive) value for the minimum number of comment element occurrences. Notice that types derived by restriction must repeat all the particle components (element declarations, model groups, and wildcards) of the base type definition that are to be included in the derived type. However, attribute declarations do not need to be repeated in the derived type definition; in this example, RestrictedPurchaseOrderType will inherit theorderDate attribute declaration from PurchaseOrderType.

This change narrows the allowable number of comment elements from a minimum of 0 to a minimum of 1. Note that all RestrictedPurchaseOrderType type elements will also be acceptable as PurchaseOrderType type elements.

To further illustrate restriction, Table 3 shows several examples of how element and attribute declarations within type definitions may be restricted (the table shows element syntax although the first three examples are equally valid attribute restrictions).

string"

specifying a type where none was previously given

(minOccurs, maxOccurs)

(minOccurs, maxOccurs)

(0, 1)

(0, 0)

exclusion of an optional component; this may also be accomplished by omitting the component's declaration from the restricted type definition

(0, 1)

(1, 1)

making an optional component required

(0, unbounded)

(1, 9)

(1, unbounded)

(1, 1)

(1, 1)

cannot further restrict minOccurs or maxOccurs

4.5 Redefining Types & Groups

In A Schema in Multiple Documents (§4.1) we described how to include definitions and declarations obtained from external schema files having the same target namespace. The [ include](#element-include) mechanism enables you to use externally created schema components "as-is", that is, without any modification. We have just described how to derive new types by extension and by restriction, and the [redefine](#element-redefine) mechanism we describe here enables you to redefine simple and complex types, groups, and attribute groups that are obtained from external schema files. Like the [include](#element-include) mechanism,[redefine](#element-redefine) requires the external components to be in the same target namespace as the redefining schema, although external components from schemas that have no namespace can also be redefined. In the latter cases, the redefined components become part of the redefining schema's target namespace.

To illustrate the [ redefine](#element-redefine) mechanism, we use it instead of the [include](#element-include) mechanism in the International Purchase Order schema,[ipo.xsd](#ipo.xsd), and we use it to modify the definition of the complex type Address contained in [ address.xsd](#address.xsd):

The `` [redefine](#element-redefine) element acts very much like the [include](#element-include) element as it includes all the declarations and definitions from the [address.xsd](#address.xsd) file. The complex type definition of Address uses the familiar extension syntax to add a country element to the definition of Address. However, note that the base type is also Address. Outside of the [ redefine](#element-redefine) element, any such attempt to define a complex type with the same name (and in the same namespace) as the base from which it is being derived would cause an error. But in this case, there is no error, and the extended definition of Address becomes the only definition of Address.

Now that Address has been redefined, the extension applies to all schema components that make use ofAddress. For example, [ address.xsd](#address.xsd) contains definitions of international address types that are derived from Address. These derivations reflect the redefined Address type, as shown in the following snippet:

.... Helen Zoe 47 Eden Street Cambridge United Kingdom CB1 1JR ....

Our example has been carefully constructed so that the redefined Address type does not conflict in any way with the types that are derived from the originalAddress definition. But note that it would be very easy to create a conflict. For example, if the international address type derivations had extended Address by adding a country element, then the redefinition of Address would be adding an element of the same name to the content model ofAddress. It is illegal to have two elements of the same name (and in the same target namespace) but different types in a content model, and so the attempt to redefine Address would cause an error. In general, [ redefine](#element-redefine) does not protect you from such errors, and it should be used cautiously.

4.6 Substitution Groups

XML Schema provides a mechanism, called substitution groups, that allows elements to be substituted for other elements. More specifically, elements can be assigned to a special group of elements that are said to be substitutable for a particular named element called the head element. (Note that the head element as well as the substitutable elementsmust be declared as global elements.) To illustrate, we declare two elements called customerComment and shipComment and assign them to a substitution group whose head element is comment, and so customerComment andshipComment can be used anyplace that we are able to use comment. Elements in a substitution group must have the same type as the head element, or they can have a type that has been derived from the head element's type. To declare these two new elements, and to make them substitutable for the comment element, we use the following syntax:

When these declarations are added to the international purchase order schema, shipComment andcustomerComment can be substituted forcomment in the instance document, for example:

.... Lapis necklace 1 99.95 ipo:shipComment Use gold wrap if possible ipo:customerComment Want this for the holidays! 1999-12-05 ....

Note that when an instance document contains element substitutions whose types are derived from those of their head elements, it is not necessary to identify the derived types using the [xsi:type](#attribute-xsitype) construction that we described in Using Derived Types in Instance Documents (§4.3).

The existence of a substitution group does not require any of the elements in that class to be used, nor does it preclude use of the head element. It simply provides a mechanism for allowing elements to be used interchangeably.

4.7 Abstract Elements and Types

XML Schema provides a mechanism to force substitution for a particular element or type. When an element or type is declared to be "abstract", it cannot be used in an instance document. When an element is declared to be abstract, a member of that element's substitution group must appear in the instance document. When an element's corresponding type definition is declared as abstract, all instances of that element must use `` [xsi:type](#attribute-xsitype) to indicate a derived type that is not abstract.

In the substitution group example we described in Substitution Groups (§4.6), it would be useful to specifically disallow use of the comment element so that instances must make use of the customerComment and shipComment elements. To declare the comment element abstract, we modify its original declaration in the international purchase order schema, [ipo.xsd](#ipo.xsd), as follows:

With comment declared as abstract, instances of international purchase orders are now only valid if they contain customerComment and shipComment elements.

Declaring an element as abstract requires the use of a substitution group. Declaring a type as abstract simply requires the use of a type derived from it (and identified by the [ xsi:type](#attribute-xsitype) attribute) in the instance document. Consider the following schema definition:

The transport element is not abstract, therefore it can appear in instance documents. However, because its type definition is abstract, it may never appear in an instance document without an `` [xsi:type](#attribute-xsitype) attribute that refers to a derived type. That means the following is not schema-valid:

because the transport element's type is abstract. However, the following is schema-valid:

because it uses a non-abstract type that is substitutable for Vehicle.

4.8 Controlling the Creation & Use of Derived Types

So far, we have been able to derive new types and use them in instance documents without any restraints. In reality, schema authors will sometimes want to control derivations of particular types, and the use of derived types in instances.

XML Schema provides a couple of mechanisms that control the derivation of types. One of these mechanisms allows the schema author to specify that for a particular complex type, new types may not be derived from it, either (a) by restriction, (b) by extension, or (c) at all. To illustrate, suppose we want to prevent any derivation of the Address type by restriction because we intend for it only to be used as the base for extended types such as USAddress and UKAddress. To prevent any such derivations, we slightly modify the original definition of Address as follows:

The restriction value of the `` [final](#attribute-final) attribute prevents derivations by restriction. Preventing derivations at all, or by extension, are indicated by the values #all and extension respectively. Moreover, there exists an optional [finalDefault](#attribute-finalDefault) attribute on the [schema](#element-schema) element whose value can be one of the values allowed for the [final](#attribute-final) attribute. The effect of specifying the [finalDefault](#attribute-finalDefault) attribute is equivalent to specifying a [final](#attribute-final) attribute on every type definition and element declaration in the schema.

Another type-derivation mechanism controls which facets can be applied in the derivation of a new simple type. When a simple type is defined, the [fixed](#attribute-facet-fixed) attribute may be applied to any of its facets to prevent a derivation of that type from modifying the value of the fixed facets. For example, we can define a Postcode simple type as:

Once this simple type has been defined, we can derive a new postal code type in which we apply a facet not fixed in the base definition, for example:

However, we cannot derive a new postal code in which we re-apply any facet that was fixed in the base definition:

In addition to the mechanisms that control type derivations, XML Schema provides a mechanism that controls which derivations and substitution groups may be used in instance documents. In Using Derived Types in Instance Documents (§4.3), we described how the derived types, USAddress and UKAddress, could be used by the shipTo and billTo elements in instance documents. These derived types can replace the content model provided by the Address type because they are derived from the Address type. However, replacement by derived types can be controlled using the [block](#attribute-block) attribute in a type definition. For example, if we want to block any derivation-by-restriction from being used in place ofAddress (perhaps for the same reason we defined Address with [final](#attribute-final)="restriction"), we can modify the original definition of Address as follows:

The restriction value on the `` [block](#attribute-block) attribute prevents derivations-by-restriction from replacing Address in an instance. However, it would not prevent UKAddress and USAddress from replacing Address because they were derived by extension. Preventing replacement by derivations at all, or by derivations-by-extension, are indicated by the values#all and extension respectively. As with [final](#attribute-final), there exists an optional [blockDefault](#attribute-blockDefault) attribute on the [schema](#element-schema) element whose value can be one of the values allowed for the [block](#attribute-block) attribute. The effect of specifying the [blockDefault](#attribute-blockDefault) attribute is equivalent to specifying a [block](#attribute-block) attribute on every type definition and element declaration in the schema.

5 Advanced Concepts III: The Quarterly Report

The home-products ordering and billing application can generate ad-hoc reports that summarize how many of which types of products have been billed on a per region basis. An example of such a report, one that covers the fourth quarter of 1999, is shown in `` [ 4Q99.xml](#Q99.xml).

Notice that in this section we use qualified elements in the schema, and default namespaces where possible in the instances.

The report lists, by number and quantity, the parts billed to various zip codes, and it provides a description of each part mentioned. In summarizing the billing data, the intention of the report is clear and the data is unambiguous because a number of constraints are in effect. For example, each zip code appears only once (uniqueness constraint). Similarly, the description of every billed part appears only once although parts may be billed to several zip codes (referential constraint), see for example part number 455-BX. In the following sections, we'll see how to specify these constraints using XML Schema.

5.1 Specifying Uniqueness

XML Schema enables us to indicate that any attribute or element value must be unique within a certain scope. To indicate that one particular attribute or element value is unique, we use the `` [unique](#element-unique) element first to "select" a set of elements, and then to identify the attribute or element "field" relative to each selected element that has to be unique within the scope of the set of selected elements. In the case of our report schema,[report.xsd](#report.xsd), the[selector](#element-selector) element's [xpath](#attribute-xpath) attribute contains an XPath expression,r: regions/ r: zip, that selects a list of all thezip elements in a report instance. Likewise, the[field](#element-field) element's[xpath](#attribute-xpath) attribute contains a second XPath expression, @code, that specifies that the code attribute values of those elements must be unique. Note that the XPath expressions limit the scope of what must be unique. The report might contain another code attribute, but its value does not have to be unique because it lies outside the scope defined by the XPath expressions. Also note that the XPath expressions you can use in the[xpath](#attribute-xpath) attribute are limited to a subset of the full XPath language defined in XML Path Language 1.0.

We can also indicate combinations of fields that must be unique. Going back to our purchase order example, suppose we want each item to have a unique combination of part number and product name. We could achieve such a constraint by specifying that for each item element, the combined values of its partNum attribute and its productName child must be unique.

To define combinations of values, we simply use multiple [field](#element-field) elements to identify all the values involved:

5.2 Defining Keys & their References

In the 1999 quarterly report, the description of every billed part appears only once. We could enforce this constraint using `` [unique](#element-unique), however, we also want to ensure that every part-quantity element listed under a zip code has a corresponding part description. We enforce the constraint using the [ key](#element-key) and [ keyref](#element-keyref) elements. The report schema, [report.xsd](#report.xsd), shows that the[key](#element-key) and [keyref](#element-keyref) constructions are applied using almost the same syntax as [unique](#element-unique). The key element applies to the number attribute value ofpart elements that are children of the parts element. This declaration of number as a key means that its value must be unique and cannot be set to nil (i.e. is not nillable), and the name that is associated with the key, pNumKey, makes the key referenceable from elsewhere.

To ensure that the part-quantity elements have corresponding part descriptions, we say that the number attribute ( <field xpath="@number"/>) of those elements (<selector xpath="r:regions/r:zip/r:part"/>) must reference the pNumKey key. This declaration of number as a keyref does not mean that its value must be unique, but it does mean there must exist a pNumKey with the same value.

As you may have figured out by analogy with [unique](#element-unique), it is possible to define combinations of [ key](#element-key) and [ keyref](#element-keyref) values. Using this mechanism, we could go beyond simply requiring the product numbers to be equal, and define a combination of values that must be equal. Such values may involve combinations of multiple value types ([string](#string), [integer](#integer), [ date](#date), etc.), provided that the order and type of the [field](#element-field) element references is the same in both the [key](#element-key) and [keyref](#element-keyref) definitions.

5.3 XML Schema Constraints vs. XML 1.0 ID Attributes

XML 1.0 provides a mechanism for ensuring uniqueness using the ID attribute and its associated attributes IDREF and IDREFS. This mechanism is also provided in XML Schema through the [ID](#ID), [IDREF](#IDREF), and [IDREFS](#IDREFS) simple types which can be used for declaring XML 1.0-style attributes. XML Schema also introduces new mechanisms that are more flexible and powerful. For example, XML Schema's mechanisms can be applied to any element and attribute content, regardless of its type. In contrast, ID is a type of attribute and so it cannot be applied to attributes, elements or their content. Furthermore, Schema enables you to specify the scope within which uniqueness applies whereas the scope of an ID is fixed to be the whole document. Finally, Schema enables you to create [ key](#element-key)s or a [ keyref](#element-keyref) from combinations of element and attribute content whereas ID has no such facility.

5.4 Importing Types

The report schema, `` [report.xsd](#report.xsd), makes use of the simple type xipo:SKU that is defined in another schema, and in another target namespace. Recall that we used [include](#element-include) so that the schema in[ipo.xsd](#ipo.xsd) could make use of definitions and declarations from [address.xsd](#address.xsd). We cannot use [include](#element-include) here because it can only pull in definitions and declarations from a schema whose target namespace is the same as the including schema's target namespace. Hence, the [include](#element-include) element does not identify a namespace (although it does require a [ schemaLocation](#attribute-schemaLocation)). The import mechanism that we describe in this section is an important mechanism that enables schema components from different target namespaces to be used together, and hence enables the schema validation of instance content defined across multiple namespaces.

To import the type SKU and use it in the report schema, we identify the namespace in which SKU is defined, and associate that namespace with a prefix for use in the report schema. Concretely, we use the [ import](#element-import) element to identify SKU's target namespace, http://www.example.com/IPO, and we associate the namespace with the prefix xipo using a standard namespace declaration. The simple type SKU, defined in the namespace http://www.example.com/IPO, may then be referenced as xipo:SKU in any of the report schema's definitions and declarations.

In our example, we imported one simple type from one external namespace, and used it for declaring attributes. XML Schema in fact permits multiple schema components to be imported, from multiple namespaces, and they can be referred to in both definitions and declarations. For example in [ report.xsd](#report.xsd) we could additionally reuse thecomment element declared in [ipo.xsd](#ipo.xsd) by referencing that element in a declaration:

Note however, that we cannot reuse the shipTo element from [ipo.xsd](#ipo.xsd), and the following is not legal because only global schema components can be imported:

In [ipo.xsd](#ipo.xsd), comment is declared as a global element, in other words it is declared as an element of the [schema](#element-schema). In contrast, shipTo is declared locally, in other words it is an element declared inside a complex type definition, specifically the PurchaseOrderType type.

Complex types can also be imported, and they can be used as the base types for deriving new types. Only named complex types can be imported; local, anonymously defined types cannot. Suppose we want to include in our reports the name of an analyst, along with contact information. We can reuse the (globally defined) complex type USAddress from [ address.xsd](#address.xsd), and extend it to define a new type called Analyst in the report schema by adding the new elements phone and email:

Using this new type we declare an element called analyst as part of the purchaseReport element declaration (declarations not shown) in the report schema. Then, the following instance document would conform to the modified report schema:

Note that the report now has both qualified and unqualified elements. This is because some of the elements (name, street, city,state and zip) are locally declared in `` [ipo.xsd](#ipo.xsd), whoseelementFormDefault is unqualified (by default). The other elements in the example are declared in [report.xsd](#report.xsd), whose elementFormDefault is set to qualified.

When schema components are imported from multiple namespaces, each namespace must be identified with a separate [import](#element-import) element. The [ import](#element-import) elements themselves must appear as the first children of the [ schema](#element-schema) element. Furthermore, each namespace must be associated with a prefix, using a standard namespace declaration, and that prefix is used to qualify references to any schema components belonging to that namespace. Finally, [import](#element-import) elements optionally contain a [schemaLocation](#attribute-schemaLocation) attribute to help locate resources associated with the namespaces. We discuss the [schemaLocation](#attribute-schemaLocation) attribute in more detail in a later section.

5.5 Any Element, Any Attribute

In previous sections we have seen several mechanisms for extending the content models of complex types. For example, a mixed content model can contain arbitrary character data in addition to elements, and for example, a content model can contain elements whose types are imported from external namespaces. However, these mechanisms provide very broad and very narrow controls respectively. The purpose of this section is to describe a flexible mechanism that enables content models to be extended by any elements and attributes belonging to specified namespaces.

To illustrate, consider a version of the quarterly report,`` [4Q99html.xml](#Q99html.xml), in which we have embedded an XHTML representation of the XML parts data. The XHTML content appears as the content of the element htmlExample, and the default namespace is changed on the outermost XHTML element (table) so that all the XHTML elements belong to the XHTML namespace, http://www.w3.org/1999/xhtml:

To permit the appearance of XHTML in the instance document we modify the report schema by declaring a new element htmlExample whose content is defined by the `` [ any](#element-any) element. In general, an [any](#element-any) element specifies that any well-formed XML is permissible in a type's content model. In the example, we require the XML to belong to the namespace http://www.w3.org/1999/xhtml, in other words, it should be XHTML. The example also requires there to be at least one element present from this namespace, as indicated by the values of [minOccurs](#attribute-minOccurs) and [maxOccurs](#attribute-maxOccurs):

The modification permits some well-formed XML belonging to the namespace http://www.w3.org/1999/xhtml to appear inside thehtmlExample element. Therefore `` [4Q99html.xml](#Q99html.xml) is permissible because there is one element which (with its children) is well-formed, the element appears inside the appropriate element (htmlExample), and the instance document asserts that the element and its content belongs to the required namespace. However, the XHTML may not actually be valid because nothing in [4Q99html.xml](#Q99html.xml) by itself can provide that guarantee. If such a guarantee is required, the value of the [processContents](#attribute-processContents) attribute should be set to strict (the default value). In this case, an XML processor is obliged to obtain the schema associated with the required namespace, and validate the XHTML appearing within thehtmlExample element.

In another example, we define a text type which is similar to the text type defined in XML Schema's introductory type library (see also Type Libraries (§5.4.1)), and is suitable for internationalized human-readable text. The text type allows an unrestricted mixture of character content and element content from any namespace, for example Ruby annotations, along with an optional xml:lang attribute. The lax value of the[processContents](#attribute-processContents) attribute instructs an XML processor to validate the element content on a can-do basis: It will validate elements and attributes for which it can obtain schema information, but it will not signal errors for those it cannot obtain any schema information.

Namespaces may be used to permit and forbid element content in various ways depending upon the value of the `` [ namespace](#attribute-namespace) attribute, as shown in Table 4:

In addition to the `` [any](#element-any) element which enables element content according to namespaces, there is a corresponding[ anyAttribute](#element-anyAttribute) element which enables attributes to appear in elements. For example, we can permit any XHTML attribute to appear as part of the htmlExample element by adding [ anyAttribute](#element-anyAttribute) to its declaration:

This declaration permits an XHTML attribute, say href, to appear in the htmlExample element. For example:

The `` [ namespace](#attribute-namespace) attribute in an [anyAttribute](#element-anyAttribute) element can be set to any of the values listed in Table 4 for the [ any](#element-any) element, and [anyAttribute](#element-anyAttribute) can be specified with a [processContents](#attribute-processContents) attribute. In contrast to an [ any](#element-any) element, [anyAttribute](#element-anyAttribute) cannot constrain the number of attributes that may appear in an element.

5.6 schemaLocation

XML Schema uses the `` [schemaLocation](#attribute-schemaLocation) and[ xsi:schemaLocation](#attribute-xsischemaLocation) attributes in three circumstances.

1. In an instance document, the attribute [ xsi:schemaLocation](#attribute-xsischemaLocation) provides hints from the author to a processor regarding the location of schema documents. The author warrants that these schema documents are relevant to checking the validity of the document content, on a namespace by namespace basis. For example, we can indicate the location of the Report schema to a processor of the Quarterly Report:

The `` [schemaLocation](#attribute-schemaLocation) attribute value consists of one or more pairs of URI references, separated by white space. The first member of each pair is a namespace name, and the second member of the pair is a hint describing where to find an appropriate schema document for that namespace. The presence of these hints does not require the processor to obtain or use the cited schema documents, and the processor is free to use other schemas obtained by any suitable means, or to use no schema at all.

A schema is not required to have a namespace (see Undeclared Target Namespaces (§3.4)) and so there is a [ noNamespaceSchemaLocation](#attribute-noNamespaceSchemaLocation) attribute which is used to provide hints for the locations of schema documents that do not have target namespaces.

2. In a schema, the [ include](#element-include) element has a required [schemaLocation](#attribute-schemaLocation) attribute, and it contains a URI reference which must identify a schema document. The effect is to compose a final effective schema by merging the declarations and definitions of the including and the included schemas. For example, in Advanced Concepts II: The International Purchase Order (§4), the type definitions of Address, USAddress, UKAddress, USState (along with their attribute and local element declarations) from [ address.xsd](#address.xsd) were added to the element declarations of purchaseOrder and comment, and the type definitions of PurchaseOrderType, Items and SKU (along with their attribute and local element declarations) from [ ipo.xsd](#ipo.xsd) to create a single schema.

3. Also in a schema, the [ import](#element-import) element has optional [namespace](#attribute-namespace) and [schemaLocation](#attribute-schemaLocation) attributes. If present, the [schemaLocation](#attribute-schemaLocation) attribute is understood in a way which parallels the interpretation of [ xsi:schemaLocation](#attribute-xsischemaLocation) in (1). Specifically, it provides a hint from the author to a processor regarding the location of a schema document that the author warrants supplies the required components for the namespace identified by the [ namespace](#attribute-namespace) attribute. To import components that are not in any target namespace, the [import](#element-import) element is used without a [namespace](#attribute-namespace) attribute (and with or without a [schemaLocation](#attribute-schemaLocation) attribute). References to components imported in this manner are unqualified.

Note that the [ schemaLocation](#attribute-schemaLocation) is only a hint and some processors and applications will have reasons to not use it. For example, an XHTML editor may have a built-in XHTML schema.

5.7 Conformance

An instance document may be processed against a schema to verify whether the rules specified in the schema are honored in the instance. Typically, such processing actually does two things, (1) it checks for conformance to the rules, a process called schema validation, and (2) it adds supplementary information that is not immediately present in the instance, such as types and default values, called infoset contributions.

The author of an instance document, such as a particular purchase order, may claim, in the instance itself, that it conforms to the rules in a particular schema. The author does this using the `` [schemaLocation](#attribute-schemaLocation) attribute discussed above. But regardless of whether a [ schemaLocation](#attribute-schemaLocation) attribute is present, an application is free to process the document against any schema. For example, a purchasing application may have the policy of always using a certain purchase order schema, regardless of any [schemaLocation](#attribute-schemaLocation) values.

Conformance checking can be thought of as proceeding in steps, first checking that the root element of the document instance has the right contents, then checking that each subelement conforms to its description in a schema, and so on until the entire document is verified. Processors are required to report what checking has been carried out.

To check an element for conformance, the processor first locates the declaration for the element in a schema, and then checks that the [targetNamespace](#attribute-targetNamespace) attribute in the schema matches the actual namespace URI of the element. Alternatively, it may determine that the schema does not have a [ targetNamespace](#attribute-targetNamespace) attribute and the instance element is not namespace-qualified.

Supposing the namespaces match, the processor then examines the type of the element, either as given by the declaration in the schema, or by an [xsi:type](#attribute-xsitype) attribute in the instance. If the latter, the instance type must be an allowed substitution for the type given in the schema; what is allowed is controlled by the [block](#attribute-block) attribute in the element declaration. At this same time, default values and other infoset contributions are applied.

Next the processor checks the immediate attributes and contents of the element, comparing these against the attributes and contents permitted by the element's type. For example, considering a shipTo element such as the one in The Purchase Order Schema (§2.1), the processor checks what is permitted for an Address, because that is the shipTo element's type.

If the element has a simple type, the processor verifies that the element has no attributes or contained elements, and that its character content matches the rules for the simple type. This sometimes involves checking the character sequence against regular expressions or enumerations, and sometimes it involves checking that the character sequence represents a value in a permitted range.

If the element has a complex type, then the processor checks that any required attributes are present and that their values conform to the requirements of their simple types. It also checks that all required subelements are present, and that the sequence of subelements (and any mixed text) matches the content model declared for the complex type. Regarding subelements, schemas can either require exact name matching, permit substitution by an equivalent element or permit substitution by any element allowed by an 'any' particle.

Unless a schema indicates otherwise (as it can for 'any' particles) conformance checking then proceeds one level more deeply by looking at each subelement in turn, repeating the process described above.