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HTML / CSS / JS

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

<xs:element name="store">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="products">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="product" type="ProductType"
                                        minOccurs="0" maxOccurs="unbounded"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
                <xs:element name="orders">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="order" type="OrderType"
                                        minOccurs="0" maxOccurs="unbounded"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>

<xs:key name="productKey">
            <xs:selector xpath="products/product"/>
            <xs:field    xpath="@id"/>
        </xs:key>

<xs:keyref name="orderItemRef" refer="productKey">
            <xs:selector xpath="orders/order/item"/>
            <xs:field    xpath="@productId"/>
        </xs:keyref>
    </xs:element>

<xs:complexType name="ProductType">
        <xs:sequence>
            <xs:element name="name"  type="xs:string"/>
            <xs:element name="price" type="xs:decimal"/>
        </xs:sequence>
        <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>

<xs:complexType name="OrderType">
        <xs:sequence>
            <xs:element name="item" maxOccurs="unbounded">
                <xs:complexType>
                    <xs:attribute name="productId" type="xs:string" use="required"/>
                    <xs:attribute name="qty"        type="xs:positiveInteger" use="required"/>
                </xs:complexType>
            </xs:element>
        </xs:sequence>
        <xs:attribute name="orderId" type="xs:string" use="required"/>
    </xs:complexType>

</xs:schema>