2007年4月7日 随笔档案

http://www.w3schools.com/schema/schema_howto.asp

What is an XML Schema?

The purpose of an XML Schema is to define the legal building blocks of an XML
document, just like a DTD.

An XML Schema:

defines elements that can appear in a document

defines attributes that can appear in a document

defines which elements are child elements

defines the order of child elements

defines the number of child elements

defines whether an element is empty or can include text

defines data types for elements and attributes

defines default and fixed values for elements and attributes

Well-Formed is not Enough

A well-formed XML document is a document that conforms to the XML syntax
rules, like:

it must begin with the XML declaration

it must have one unique root element

start-tags must have matching end-tags

elements are case sensitive

all elements must be closed

all elements must be properly nested

all attribute values must be quoted

entities must be used for special characters

The element may contain some attributes. A schema declaration
often looks something like this:

…
…

The following fragment:

xmlns:xs=”http://www.w3.org/2001/XMLSchema”

indicates that the elements and data types used in the schema come from the
“http://www.w3.org/2001/XMLSchema” namespace. It also specifies that the
elements and data types that come from the “http://www.w3.org/2001/XMLSchema”
namespace should be prefixed with xs:

This fragment:

targetNamespace=”http://www.w3schools.com”

indicates that the elements defined by this schema (note, to, from, heading,
body.) come from the “http://www.w3schools.com” namespace.

This fragment:

xmlns=”http://www.w3schools.com”

indicates that the default namespace is “http://www.w3schools.com”.

This fragment:

elementFormDefault=”qualified”

indicates that any elements used by the XML instance document which were
declared in this schema must be namespace qualified.

What is a Simple Element?

A simple element is an XML element that can contain only text. It cannot
contain any other elements or attributes.

Defining a Simple Element

The syntax for defining a simple element is:

XML Schema has a lot of built-in data types. The most common types are:

xs:string

xs:decimal

xs:integer

xs:boolean

xs:date

xs:time

Default and Fixed Values for Simple Elements

Simple elements may have a default value OR a fixed value specified.

A default value is automatically assigned to the element when no other value
is specified.

In the following example the default value is “red”:

A fixed value
is also automatically assigned to the element, and you cannot specify another
value.

In the following example the fixed value is “red”:

How to Define an Attribute?

The syntax for defining an attribute is:

Restrictions are used to define acceptable values for XML
elements or attributes. Restrictions on XML elements are called facets.

Restrictions on Values

The following example defines an element called “age” with a restriction. The
value of age cannot be lower than 0 or greater than 120:

Restrictions on a Set of Values

To limit the content of an XML element to a set of acceptable values, we
would use the enumeration constraint.

The example below defines an element called “car” with a restriction. The
only acceptable values are: Audi, Golf, BMW:

The example above could also have been written like this:

Note: In this case the type “carType” can be used by other elements
because it is not a part of the “car” element.

Restrictions on a Series of Values

To limit the content of an XML element to define a series of numbers or
letters that can be used, we would use the pattern constraint.

The example below defines an element called “letter” with a restriction. The
only acceptable value is ONE of the LOWERCASE letters from a to z:

The next example defines an element called “initials” with a restriction. The
only acceptable value is THREE of the UPPERCASE letters from a to z:

The next example also defines an element called “initials” with a
restriction. The only acceptable value is THREE of the LOWERCASE OR UPPERCASE
letters from a to z:

The next example defines an element called “choice” with a restriction. The
only acceptable value is ONE of the following letters: x, y, OR z:

The next example defines an element called “prodid” with a restriction. The
only acceptable value is FIVE digits in a sequence, and each digit must be in a
range from 0 to 9:

Other Restrictions on a Series of Values

The example below defines an element called “letter” with a restriction. The
acceptable value is zero or more occurrences of lowercase letters from a to
z:

The next example also defines an element called “letter” with a restriction.
The acceptable value is one or more pairs of letters, each pair consisting of a
lower case letter followed by an upper case letter. For example, “sToP” will be
validated by this pattern, but not “Stop” or “STOP” or “stop”:

The next example defines an element called “gender” with a restriction. The
only acceptable value is male OR female:

The next example defines an element called “password” with a restriction.
There must be exactly eight characters in a row and those characters must be
lowercase or uppercase letters from a to z, or a number from 0 to 9:

Restrictions on Whitespace Characters

To specify how whitespace characters should be handled, we would use the
whiteSpace constraint.

This example defines an element called “address” with a restriction. The
whiteSpace constraint is set to “preserve”, which means that the XML processor
WILL NOT remove any white space characters:

This example also defines an element called “address” with a restriction. The
whiteSpace constraint is set to “replace”, which means that the XML processor
WILL REPLACE all white space characters (line feeds, tabs, spaces, and carriage
returns) with spaces:

This example also defines an element called “address” with a restriction. The
whiteSpace constraint is set to “collapse”, which means that the XML processor
WILL REMOVE all white space characters (line feeds, tabs, spaces, carriage
returns are replaced with spaces, leading and trailing spaces are removed, and
multiple spaces are reduced to a single space):

Restrictions on Length

To limit the length of a value in an element, we would use the length,
maxLength, and minLength constraints.

This example defines an element called “password” with a restriction. The
value must be exactly eight characters:

This example defines another element called “password” with a restriction.
The value must be minimum five characters and maximum eight characters:

Restrictions for Datatypes

Constraint
Description

enumeration
Defines a list of acceptable values

fractionDigits
Specifies the maximum number of decimal places allowed. Must be
equal to or greater than zero

length
Specifies the exact number of characters or list items allowed.
Must be equal to or greater than zero

maxExclusive
Specifies the upper bounds for numeric values (the value must be
less than this value)

maxInclusive
Specifies the upper bounds for numeric values (the value must be
less than or equal to this value)

maxLength
Specifies the maximum number of characters or list items allowed.
Must be equal to or greater than zero

minExclusive
Specifies the lower bounds for numeric values (the value must be
greater than this value)

minInclusive
Specifies the lower bounds for numeric values (the value must be
greater than or equal to this value)

minLength
Specifies the minimum number of characters or list items allowed.
Must be equal to or greater than zero

pattern
Defines the exact sequence of characters that are acceptable

totalDigits
Specifies the exact number of digits allowed. Must be greater
than zero

whiteSpace
Specifies how white space (line feeds, tabs, spaces, and carriage
returns) is handled

What is a Complex Element?

A complex element is an XML element that contains other elements and/or
attributes.

There are four kinds of complex elements:

empty elements

elements that contain only other elements

elements that contain only text

elements that contain both other elements and text

How to Define a Complex Element

Look at this complex XML element, “employee”, which contains only other
elements:

John
Smith

We can define a complex element in an XML Schema two different ways:

1. The “employee” element can be declared directly by naming the element,
like this:

If you use the method described above, only the “employee” element can use
the specified complex type. Note that the child elements, “firstname” and
“lastname”, are surrounded by the indicator. This means that
the child elements must appear in the same order as they are declared. You will
learn more about indicators in the XSD Indicators chapter.

2. The “employee” element can have a type attribute that refers to the name
of the complex type to use:

You can also base a complex element on an existing complex element and add
some elements, like this:

Complex Types with Mixed Content

An XML element, “letter”, that contains both text and other elements:

Dear Mr.John Smith.
Your order 1032
will be shipped on 2001-07-13.

The following schema declares the “letter” element:

Note: To enable character data to appear between the child-elements of
“letter”, the mixed attribute must be set to “true”. The tag
means that the elements defined (name, orderid and shipdate) must appear in that
order inside a “letter” element.

We can control HOW elements are to be used in documents with
indicators.

Indicators

There are seven indicators:

Order indicators:

All

Choice

Sequence

Occurrence indicators:

maxOccurs

minOccurs

Group indicators:

Group name

attributeGroup name

All Indicator

The indicator specifies that the child elements can appear in any
order, and that each child element must occur only once:

Note: When using the indicator you can set the
indicator to 0 or 1 and the indicator can
only be set to 1

Choice Indicator

The indicator specifies that either one child element or
another can occur:

Sequence Indicator

The indicator specifies that the child elements must appear
in a specific order:

Occurrence Indicators

Occurrence indicators are used to define how often an element can occur.

Note: For all “Order” and “Group” indicators (any, all, choice,
sequence, group name, and group reference) the default value for maxOccurs and
minOccurs is 1.

maxOccurs Indicator

The indicator specifies the maximum number of times an
element can occur:

The example above indicates that the “child_name” element can occur a minimum
of one time (the default value for minOccurs is 1) and a maximum of ten times in
the “person” element.

minOccurs Indicator

The indicator specifies the minimum number of times an
element can occur:

The example above indicates that the “child_name” element can occur a minimum
of zero times and a maximum of ten times in the “person” element.

Tip: To allow an element to appear an unlimited number of times, use
the maxOccurs=”unbounded” statement

Group Indicators

Group indicators are used to define related sets of elements.

Element Groups

Element groups are defined with the group declaration, like this:

…

You must define an all, choice, or sequence element inside the group
declaration. The following example defines a group named “persongroup”, that
defines a group of elements that must occur in an exact sequence:

After you have defined a group, you can reference it in another definition,
like this:

Attribute Groups

Attribute groups are defined with the attributeGroup declaration, like
this:

…

The following example defines an attribute group named “personattrgroup”:

After you have defined an attribute group, you can reference it in another
definition, like this:

The element enables us to extend the XML document
with elements not specified by the schema!

The Element

The element enables us to extend the XML document with elements
not specified by the schema.

The following example is a fragment from an XML schema called “family.xsd”.
It shows a declaration for the “person” element. By using the
element we can extend (after ) the content of “person” with any
element:

Now we want to extend the “person” element with a “children” element. In this
case we can do so, even if the author of the schema above never declared any
“children” element.

Look at this schema file, called “children.xsd”:

The XML file below (called “Myfamily.xml”), uses components from two
different schemas; “family.xsd” and “children.xsd”:

Hege
Refsnes

Cecilie

Stale
Refsnes

The XML file above is valid because the schema “family.xsd” allows us to
extend the “person” element with an optional element after the “lastname”
element.

The and elements are used to make EXTENSIBLE
documents! They allow documents to contain additional elements that are not
declared in the main XML schema.

The element enables us to extend the XML
document with attributes not specified by the schema!

The Element

The element enables us to extend the XML document with
attributes not specified by the schema.

The following example is a fragment from an XML schema called “family.xsd”.
It shows a declaration for the “person” element. By using the
element we can add any number of attributes to the “person”
element:

Now we want to extend the “person” element with a “gender” attribute. In this
case we can do so, even if the author of the schema above never declared any
“gender” attribute.

Look at this schema file, called “attribute.xsd”:

The XML file below (called “Myfamily.xml”), uses components from two
different schemas; “family.xsd” and “attribute.xsd”:

Hege
Refsnes

Stale
Refsnes

The XML file above is valid because the schema “family.xsd” allows us to add
an attribute to the “person” element.

The and elements are used to make EXTENSIBLE
documents! They allow documents to contain additional elements that are not
declared in the main XML schema.