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Tutorials / W3C XSD Schema Tutorial / XSD Tutorial - Part 3 - Extending Existing Types
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    XSD Tutorial - Part 3 - Extending Existing Types
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    It is often useful to be able to take the definition for an existing entity, and extend it to add more specific information. In most development languages we would call this inheritance or sub classing. The same concepts also exist in the XSD standard. This allows us to take an existing type definition and extend it. We can also restrict an existing type (although this behavior has no real parallel in most development languages).

    Extending an Existing ComplexType

    It is possible to take an existing <xs:complexType> and extend it. Let's see how this may be useful with an example.

    Looking at the AddressType that we defined earlier (in part 1), let's assume our company has now gone international and we need to capture country specific addresses. In this case we need specific information for UK addresses (County and Postcode), and for US addresses (State and ZipCode).

    So we can take our existing definition of address and extend it as follows

    <xs:complexType name="UKAddressType"> 
            <xs:extension base="AddressType"> 
                    <xs:element name="County" type="xs:string"/> 
                    <xs:element name="Postcode" type="xs:string"/> 
    <xs:complexType name="USAddressType"> 
            <xs:extension base="AddressType"> 
                    <xs:element name="State" type="xs:string"/> 
                    <xs:element name="Zipcode" type="xs:string"/> 
    This is clearer when viewed graphically. But basically it is saying - we are defining a new <xs:complexType> called "USAddressType", this extended's the existing type "AddressType", and adds to it a sequence containing the elements "State", and "Zipcode".
    There are 2 new things here the <xs:extension> element and the <xs:complexContent> element, we'll get to these shortly.

    We can now use these new types as follows:

    <xs:element name="UKAddress" type="UKAddressType"/> 
    <xs:element name="USAddress" type="USAddressType"/>

    Some Sample XML for these elements may look like this.

    <Line1>34 thingy street</Line1>



    <Line1>234 Lancaster Av</Line1>


    The last example showed how to take an existing <xs:complexType> definition, and extend it to create new types. The new construct <xs:extension> indicates that we are extending an existing type, and specifies the type itself. But there is another option here, instead of adding to the type, we could restrict it.

    Restricting an Existing ComplexType

    Taking the same AddressType example, we can create a new type called "InternalAddressType". Let's assume "InternalAddressType" only needs Address->Line1.

     <xs:complexType name="InternalAddressType">
    <xs:restriction base="AddressType">
    <xs:element name="Line1" type="xs:string" />

    We are defining a new type "InternalAddressType". The <xs:restriction> element says we are restricting the existing type "AddressType" , and we are only allowing the existing child element "Line1" to be used in this new definition.
    Note: Because we are restricting an existing type the only definitions that can appear in the <xs:restriction> are a sub set of the ones defined in the base type "AddressType". They must also be enclosed in the same compositor (in this case a sequence) and appear in the same order.

    We can now use this new type as follows:

    <xs:element name="InternalAddress" type="InternalAddressType"/>

    Some Sample XML for this element may look like this.

    <Line1>Desk 4, Second Floor/<Line1>

    Note: The <xs:complexContent> element is just a container for the extension or restriction - we can largely ignore it for now.

    Use of Extended/Restricted Types

    We have just shown how we can create new types based on existing one. This in itself is pretty useful, and will potentially reduce the amount of complexity in your schemas, making them easier to maintain and understand. However there is an aspect to this that has not yet been covered. In the above examples we created 3 new types (UKAddressType, USAddressType and InternalAddressType), all based on AddressType.

    So, if we have an element that specifies it's of type UKAddressType, then that is what must appear in the XML document.
    But if an element specifies it's of type "AddressType", then any of the 4 types can appear in the XML document (UKAddressType, USAddressType, InternalAddressType or AddressType).
    The thing to consider now is, how will the XML parser know which type you meant to use, surely it needs to know otherwise it can not do proper validation?

    Well, it knows because if you want to use a type other than the one explicitly specified in the schema (in this case AddressType) then you have to let the parser know which type your using. This is done in the XML document using the xsi:type attribute.

    Let's look at an example.

      <xs:element name="Person">
    <xs:element name="Name" type="xs:string" />
    <xs:element name="HomeAddress" type="AddressType" />

    This sample XML is the kind of thing you would expect to see.

    <?xml version="1.0"?>
    <Line1>22 whatever place, someplace</Line1> <Line2>sometown, ss1 6gy </Line2> </HomeAddress>

    But the following is also valid.

    <?xml version="1.0"?>
    <Person xmlns:xsi="">
    <HomeAddress xsi:type="USAddressType">
    <Line1>234 Lancaseter Av</Line1>

    Let's look at that in more detail.

    But more about namespace's in the next section.

    Extending Simple Types

    There are 3 ways in which a simpleType can be extended; Restriction, List or Union. The most common is Restriction, but we will cover the other 2 as well.


    Restriction is a way to constrain an existing type definition. We can apply a restriction to the built in data types xs:string, xs:integer, xs:date, etc or ones we create ourselves.

    Here we are defining a restriction the existing type "string", we are applying a regular expression to it, to limit the values it can take.

    <xs:simpleType name="LetterType">
        <xs:restriction base="xs:string">
            <xs:pattern value="[a-zA-Z]"/>
    Shown graphically in Liquid XML Studio as follows

    Let’s go through this line by line.
    1. A <simpleType> tag is used to define a our new type, we must give the type a unique name - in this case "LetterType"
    2. We are restricting an existing type - so the tag is <restriction> (you can also extend an existing type - but more about this later). We are basing our new type on a string so type="xs:string"
    3. We are applying a restriction in the form of a Regular expression, this is specified using the <pattern> element. The regular expression means the data must contain a single lower or upper case letter a through to z.
    4. closing tag for the restriction
    5. closing tag for the simple type

    Restrictions may also be referred to as Facets. For a complete list see the XSD Standard, but to give you an idea, here are a few to get you started.



    Syntax explained

    This specifies the minimum and maximum length allowed

    Must be 0 or greater

    <xs:minLength value="3">
    <xs:maxLength value="8">

    In this example the length must be between 3 and 8

    The lower and upper range for numerical values

    The value must be less than or equal to, greater than or equal to

    <xs:minInclusive value="0">
    <xs:maxInclusive value="10">

    The value must be between 0 and 10

    The lower and upper range for numerical values

    The value must be less than or greater than

    <xs:minExclusive value="0">
    <xs:maxExclusive value="10">

    The value must be between 1 and 9

    The exact number of characters allowed

    <xs:length value="30">

    The length must not be more than 30

    Exact number of digits allowed

    <xs:totalDigits value="9">

    Can not have more than 9 digits

    A list of values allowed

    <xs:enumeration value="Hippo"/> <xs:enumeration value="Zebra"/> <xs:enumeration value="Lion"/>

    The only permitted values are Hippo, Zebra or Lion

    The number of decimal places allowed (must be >= 0)

    <xs:fractionDigits value="2"/>

    The value has to be to 2 d.p.

    This defines how whitespace will be handled. Whitespace is line feeds, carriage returns, tabs, spaces, etc.

    <xs:whitespace value=   "preserve"/>
    <xs:whitespace value=   "replace"/>
    <xs:whitespace value=   "collapse"/>

    Preserve - Keeps whitespaces
    Replace - Replaces all whitespace with a space
    Collapse - Replaces whitespace characters with a space, then if there are multiple spaces together then they will be reduced to one space.

    Pattern determines what characters are allowed and in what order. These are regular expressions and there is a complete list at:

    <xs:pattern value="[0-999]"/>

    [0-999] - 1 digit only between 0 and 999

    [0-99][0-99][0-99] - 3 digits all have to be between 0 and 99

    [a-z][0-10][A-Z] - 1st digit has to be between a and z and 2nd digit has to be between 0 and 10 and the 3rd digit is between A and Z. These are case sensitive.

    [a-zA-Z] - 1 digit that can be either lower or uppercase A – Z

    [123] - 1 digit that has to be 1, 2 or 3

    ([a-z])* - Zero or more occurrences of a to z

    ([q][u])+ - Looking for a pair letters that satisfy the criteria, in this case a q followed by a u

    ([a-z][0-999])+ - As above, looking for a pair where the 1st digit is lowercase and between a and z, and the 2nd digit is between 0 and 999, for example a1, c99, z999, f45

    [a-z0-9]{8} - Must be exactly 8 characters in a row and they must be lowercase a to z or number 0 to 9.

    It is important to note that not all facets are valid for all data types - for example, maxInclusive has no meaning when applied to a string. For the combinations of facets that are valid for a given data type refer to the XSD standard.


    A union is a mechanism for combining 2 or more different data types into one.

    The following defines 2 simple types "SizeByNumberType" all the positive integers up to 21 (e.g. 10, 12, 14), and "SizeByStringNameType" the values small, medium and large.

    <xs:simpleType name="SizeByNumberType">
        <xs:restriction base="xs:positiveInteger"> 
            <xs:maxInclusive value="21"/>
    <xs:simpleType name="SizeByStringNameType"> 
        <xs:restriction base="xs:string">
            <xs:enumeration value="small"/>
            <xs:enumeration value="medium"/> 
            <xs:enumeration value="large"/>

    We can then define a new type called "USClothingSizeType", we define this as a union of the types "SizeByNumberType" and "SizeByStringNameType" (although we can add any number of types, including the built in types - separated by whitespace).

    <xs:simpleType name="USClothingSizeType"> 
        <xs:union memberTypes="SizeByNumberType SizeByStringNameType" />

    This means the type can contain any of the values that the 2 members can take (e.g. 1, 2, 3, ...., 20, 21, small, medium, large).

    This new type can then be used in the same way as any other <xs:simpleType>


    A list allows the value (in the XML document) to contain a number of valid values separated by whitespace.

    A List is constructed in a similar way to a Union. The difference being that we can only specify a single type. This new type can contain a list of values that are defined by the itemType  property. The values must be whitespace separated. So a valid value for this type would be "5 9 21".

    <xs:simpleType name="SizesinStockType">
        <xs:list itemType="SizeByNumberType" />