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This chapter is from the book

This chapter is from the book

8.3. Simple type restrictions

Every simple type is a restriction of another simple type, known as its base type. It is not possible to extend a simple type, except by adding attributes which results in a complex type. This is described in Section 13.4.1 on p. 306.

Every new simple type restricts the value space of its base type in some way. Example 8–3 shows a definition of DressSizeType that restricts the built-in type integer.

Example 8–3. Deriving a simple type from a built-in simple type

<xs:simpleType name="DressSizeType">
  <xs:restriction base="xs:integer">
    <xs:minInclusive value="2"/>
    <xs:maxInclusive value="18"/>
    <xs:pattern value="\d{1,2}"/>

Simple types may also restrict user-derived simple types that are defined in the same schema document, or even in a different schema document. For example, you could further restrict DressSizeType by defining another simple type, MediumDressSizeType, as shown in Example 8–4.

A simple type restricts its base type by applying facets to restrict its values. In Example 8–4, the facets minInclusive and maxInclusive are used to restrict the value of MediumDressSizeType to be between 8 and 12 inclusive.

Example 8–4. Deriving a simple type from a user-derived simple type

<xs:simpleType name="MediumDressSizeType">
  <xs:restriction base="DressSizeType">
    <xs:minInclusive value="8"/>
    <xs:maxInclusive value="12"/>

8.3.1. Defining a restriction

The syntax for a restriction element is shown in Table 8-3. You must specify one base type either by using the base attribute or by defining the simple type anonymously using a simpleType child. The option of using a simpleType child is generally only useful when restricting list types, as described in Section 10.3.3 on p. 190.

Table 8-3. XSD Syntax: simple type restriction





Attribute name





Unique ID.



Simple type that is being restricted; either a base attribute or a simpleType child is required.


annotation?, simpleType?, (minExclusive | minInclusive | maxExclusive | maxInclusive | length | minLength | maxLength | totalDigits | fractionDigits | enumeration | pattern | whiteSpace | 1.1assertion | 1.1explicitTimezone | 1.1{any element in another namespace})*

Within a restriction element, you can specify any of the facets, in any order. However, the only facets that may appear more than once in the same restriction are pattern, enumeration, and assertion. It is legal to define a restriction that has no facets specified. In this case, the derived type allows the same values as the base type.

8.3.2. Overview of the facets

The available facets are listed in Table 8-4.

Table 8-4. Facets




Value must be greater than x.


Value must be greater than or equal to x.


Value must be less than or equal to x.


Value must be less than x.


The length of the value must be equal to x.


The length of the value must be greater than or equal to x.


The length of the value must be less than or equal to x.


The number of significant digits must be less than or equal to x.


The number of fractional digits must be less than or equal to x.


The schema processor should either preserve, replace, or collapse whitespace depending on x.


x is one of the valid values.


x is one of the regular expressions that the value may match.


The time zone part of the date/time value is required, optional, or prohibited depending on x.


The value must conform to a constraint in the XPath expression.

The syntax for applying a facet is shown in Table 8-5. All facets (except assertion) must have a value attribute, which has different valid values depending on the facet. Most facets may also have a fixed attribute, as described in Section 8.3.4 on p. 140.

Table 8-5. XSD Syntax: facet


minExclusive, minInclusive, maxExclusive, maxInclusive, length, minLength, maxLength, totalDigits, fractionDigits, enumeration, pattern, whiteSpace, 1.1explicitTimezone



Attribute name





Unique ID.



Value of the restricting facet.



Value of the restricting facet. various Whether the facet is fixed and therefore cannot be restricted further (see Section 8.3.4); not applicable for pattern, enumeration.



† The assertion facet has a different syntax that is described in Table 14–1.

Certain facets are not applicable to some types. For example, it does not make sense to apply the fractionDigits facet to a character string type. There is a defined set of applicable facets for each of the built-in types.1 If a facet is applicable to a built-in type, it is also applicable to atomic types that are derived from it. For example, since the length facet is applicable to string, if you derive a new type from string, the length facet is also applicable to your new type. Section 8.4 on p. 142 describes each of the facets in detail and lists the built-in types to which the facet can apply.

8.3.3. Inheriting and restricting facets

When a simple type restricts its base type, it inherits all of the facets of its base type, its base type’s base type, and so on back through its ancestors. Example 8–4 showed a simple type MediumDressSizeType whose base type is DressSizeType. DressSizeType has a pattern facet which restricts its value space to one- or two-digit numbers. Since MediumDressSizeType inherits all of the facets from DressSizeType, this same pattern facet applies to MediumDressSizeType also. Example 8–5 shows an equivalent definition of MediumDressSizeType where it restricts integer and has the pattern facet applied.

Example 8–5. Effective definition of MediumDressSizeType

<xs:simpleType name="MediumDressSizeType">
  <xs:restriction base="xs:integer">
    <xs:minInclusive value="8"/>
    <xs:maxInclusive value="12"/>
    <xs:pattern value="\d{1,2}"/>

Sometimes a simple type definition will include facets that are also specified for one of its ancestors. In Example 8–4, MediumDressSizeType includes minInclusive and maxInclusive, which are also applied to its base type, DressSizeType. The minInclusive and maxInclusive facets of MediumDressSizeType (whose values are 8 and 12, respectively) override those of DressSizeType (2 and 18, respectively).

It is a requirement that the facets of a derived type (in this case MediumDressSizeType) be more restrictive than those of the base type. In Example 8–6, we define a new restriction of DressSizeType, called SmallDressSizeType, and set minInclusive to 0. This type definition is illegal, because it attempts to expand the value space by allowing 0, which was not valid for DressSizeType.

Example 8–6. Illegal attempt to extend a simple type

<xs:simpleType name="SmallDressSizeType">
  <xs:restriction base="DressSizeType">
    <xs:minInclusive value="0"/>
    <xs:maxInclusive value="6"/>

This rule also applies when you are restricting the built-in types. For example, the short type has a maxInclusive value of 32767. It is illegal to define a restriction of short that sets maxInclusive to 32768.

Although enumeration facets can appear multiple times in the same type definition, they are treated in much the same way. If both a derived type and its ancestor have a set of enumeration facets, the values of the derived type must be a subset of the values of the ancestor. An example of this is provided in Section 8.4.4 on p. 145.

Likewise, the pattern facets specified in a derived type must allow a subset of the values allowed by the ancestor types. A schema processor will not necessarily check that the regular expressions represent a subset; instead, it will validate instances against the patterns of both the derived type and all the ancestor types, effectively taking the intersection of the pattern values.

8.3.4. Fixed facets

When you define a simple type, you can fix one or more of the facets. This means that further restrictions of this type cannot change the value of the facet. Any of the facets may be fixed, with the exception of pattern, enumeration, and assertion. Example 8–7 shows our DressSizeType with fixed minInclusive and maxInclusive facets, as indicated by a fixed attribute set to true.

Example 8–7. Fixed facets

<xs:simpleType name="DressSizeType">
  <xs:restriction base="xs:integer">
    <xs:minInclusive value="2" fixed="true"/>
    <xs:maxInclusive value="18" fixed="true"/>
    <xs:pattern value="\d{1,2}"/>

With this definition of DressSizeType, it would have been illegal to define the MediumDressSizeType as shown in Example 8–4 because it attempts to override the minInclusive and maxInclusive facets which are now fixed. Some of the built-in types have fixed facets that cannot be overridden. For example, the built-in type integer has its fractionDigits facet fixed at 0, so it is illegal to derive a type from integer and specify a fractionDigits that is not 0. Design hint: When should I fix a facet?

Fixing facets makes your type less flexible and discourages other schema authors from reusing it. Keep in mind that any types that may be derived from your type must be more restrictive, so you are not at risk that your type will be dramatically changed if its facets are unfixed.

A justification for fixing facets might be that changing that facet value would significantly alter the meaning of the type. For example, suppose you want to define a simple type that represents price. You define a Price type and fix the fractionDigits at 2. This still allows other schema authors to restrict Price to define other types, for example, by limiting it to a certain range of values. However, they cannot modify the fractionDigits of the type, because this would result in a type not representing a price in dollars and cents.

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