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FIGURE 25-4 A slightly expanded XML purchase order document
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< xml version="1.0" > <purchaseOrder> <customerNumber>2117</customerNumber> <orderNumber>112961</orderNumber> <orderDate>2007-12-17</orderDate> <repNumber>106</repNumber> <terms ship="ground" bill="Net30"></terms> <orderItem> <mfr>REI</mfr> <product>2A44L</product> <qty>7</qty> <amount>31500.00</amount> </orderItem> <orderItem> <mfr>ACI</mfr> <product>41003</product> <qty>10</qty> <amount>6520.00</amount> </orderItem> </purchaseOrder>
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Many useful real-world XML documents do not map neatly into single rows of a table. Figure 25-4 shows a simple extension of the purchase order XML document from Figure 25-3, which supports the typical real-world requirement that a purchase order may contain multiple line items. How should this XML document be unmarshaled into the sample database One solution is to make each line item from the purchase order into a separate row of the ORDERS table. (Ignore for the moment that each row in the ORDERS table must contain a unique order number because the order number is the primary key.) This would result in some duplication of data, since the same order number, order date, customer number, and salesperson number will appear in several rows. It would also make marshaling the data to reconstitute the document more complex the DBMS would have to know that all of the rows with the same order number should be marshaled into one purchase order XML document with multiple line items. Clearly, the marshaling/unmarshaling of even this simple document requires a more complex mapping. The multiline purchase order merely scratches the surface of marshaling and unmarshaling XML documents. The more general situation is shown in Figure 25-5, where the DBMS must unmarshal an XML document into multiple rows of multiple, interrelated tables. To marshal the document, the DBMS must exercise the relationships between the tables to find the related rows and recompose the XML hierarchy. The underlying reason for this complexity is the mismatch between XML s natural hierarchical structure and the flat, normalized, row/column structure of a relational database.
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FIGURE 25-5
XML marshaling and unmarshaling for a database
Marshaling and unmarshaling are both simplified and made more complex if a DBMS supports object-relational extensions such as structured data types. The translation to and from XML can be simpler because individual columns of a table can now have their own hierarchical structure. A higher-level XML element (such as a billing address composed of street, city, state, country, and postal code elements) can be mapped into a corresponding column with an abstract ADDRESS data type, with its own internal hierarchy. However, the translation to and from XML now involves more decisions in the database design, trading off the marshaling/unmarshaling simplicity of structured data types against the flexibility of a flattened row/column approach. Several commercial products are beginning to offer marshaling/unmarshaling capabilities, or have announced plans to provide this capability in future releases. The performance overhead of this translation can be very substantial, and it remains to be seen how popular these capabilities will be in practice. However, if an application is handling external data in XML form, the translation between XML and SQL data must occur at some point, and translation within the DBMS itself may be the most efficient approach.
XML and Metadata
One of the most powerful qualities of the relational model is its very rigid support for data types and data structure, implemented by the definitions of tables, columns, primary keys, foreign keys, and constraints. In addition, as shown in 16, the system catalog of a relational database contains metadata, or data about the data in the database. By querying the system catalog, you can discover the structure of the database, including the data types of its columns, the columns that compose its tables, and intratable relationships.
25:
SQL and XML
In contrast, XML documents by themselves provide only very limited metadata. They impose a hierarchical element structure on their data, but the only real data about the structure is the names of the elements and attributes. An XML document can be well formed and still have quite an irregular structure. For example, there is nothing to prevent a wellformed XML document from having a named element that contains text data in one instance and subelements in another instance, or a named attribute that has an integer value for one element and a date value for another. Clearly, a document with this structure, while it may be well formed, does not represent data that is easily transformed to and from a relational database. When using XML for data processing documents, stronger support for data types and rigid structure is needed. XML standards and products have addressed this need in multiple ways during the short history of XML technologies. These include Document Type Definition (DTD) A part of the original XML 1.0 specification, Document Type Definitions provided a way to specify and restrict the structure of a document. XML parsers can examine an XML document in the context of a DTD and determine whether it is a valid document (i.e., whether it conforms to the DTD restrictions). XML-Data Submitted to the W3C in 1998, XML-Data was an early attempt to address some of the deficiencies in the DTD scheme. It never received W3C endorsement, but many of its ideas have carried forward into the XML Schema specification. Microsoft adapted its own form of XML-Data, called XML-Data Reduced (XDR), and implemented it as part of its BizTalk integration server and Internet Explorer 5.0 browser. The energy around the XML-Data proposal shifted in late 1999 and 2000 to the XML Schema proposal. XML Schema A stand-alone specification that became a W3C recommendation in May 2001, XML Schema built on and extended the ideas in XML-Data. XML Schema provides much more rigorous data type support and has the advantage that the schema definition (the document metadata) is itself expressed as an XML document, in much the same way that relational database metadata is provided via a standard relational table structure. Industry group standards As mentioned earlier, various industry groups have banded together to define XML standards for specific types of documents that are important for data exchange within their industry. For example, financial services firms are working on standards to describe financial instruments (stocks, bonds, etc.) and market data. Manufacturing firms are working on standards to describe purchase order documents, order confirmations, and the like. These standards for specific industry-oriented documents are usually built on generic standards, such as DTD and XML Schema. The area of XML metadata and document type standards is evolving rapidly. The W3C provides a frequently updated web site at http://www.w3.org, which provides access to the various XML-related standards and information about their status. You can find information about industry-specific standards at http://www.xml.org, a site organized and hosted by the Organization for the Advancement of Structured Information Systems (OASIS). The site contains a registry of XML-based standards, classified by industry.
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