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

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

An XML Schema:

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

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

The following fragment:

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:

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

This fragment:

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

This fragment:

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

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

The syntax for defining a simple element is:

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

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":

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.

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

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.

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:

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:

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):

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:

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

There are four kinds of complex elements:

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

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:

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

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.

There are seven indicators:

Order indicators:

Occurrence indicators:

Group indicators:

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 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 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 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":

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 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":

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.