<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <!-- Root element -->
    <xs:element name="catalog">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="book" type="bookType" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

    <!-- Book type definition -->
    <xs:complexType name="bookType">
        <xs:sequence>
            <xs:element name="title" type="xs:string"/>
            <xs:element name="author" type="xs:string"/>
            <xs:element name="year" type="xs:positiveInteger"/>
            <xs:element name="price" type="xs:decimal"/>
            <xs:element name="category" type="categoryType"/>
        </xs:sequence>
        <xs:attribute name="id" type="xs:positiveInteger" use="required"/>
    </xs:complexType>

    <!-- Category restriction -->
    <xs:simpleType name="categoryType">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Fiction"/>
            <xs:enumeration value="Classic"/>
            <xs:enumeration value="Science Fiction"/>
            <xs:enumeration value="Mystery"/>
            <xs:enumeration value="Biography"/>
        </xs:restriction>
    </xs:simpleType>

</xs:schema>