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When exchanged between applications or stored in a file or in a DBMS CLOB column, XML documents are always in text form. This makes the contents very portable, but unwieldy for computer programs to handle. An XML parser is a piece of computer software that translates XML documents from their text form to a more program-friendly, internal representation. Any SQL-based DBMS that supports XML will have an XML
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parser as part of its software, for its own use in processing XML. If the DBMS brand supports CLOBs, it can provide further integration with XML by allowing an XML parser to operate directly on the CLOB column contents. There are two popular types of XML parsers, which support two styles of XML processing: I Document Object Model (DOM). DOM parsers transform an XML document into a hierarchical tree structure within a computer s main memory. A program can than make calls to the DOM API to navigate through the tree, moving up and down or sequentially through the element hierarchy. The DOM API makes the element structure of an XML document easily accessible to programmers and simplifies random access to portions of the document. I Simple XPI for XML (SAX). SAX parsers transform an XML document into a series of callbacks to a program, which inform the program of each part of the XML document as it is encountered. A program can be structured to take certain actions when the beginning of a document section is encountered, or when a particular attribute is encountered. The SAX API imposes a more sequential style of processing on a program using it. The API s callback style matches well with an event-driven program structure. Either type of XML parser will validate that an XML document is well formed, and can also validate an XML document against a schema, as described in the XML Schema section later in this chapter. A DOM parser is practical when the size of the stored XML document is fairly small; it will require double the memory space of the text XML document, because it generates a second, tree-structured representation of the entire document. For very large documents, a SAX parser makes it easy to process documents in small, discrete pieces. However, the fact that the entire document is not available at one time may require a program to make multiple passes through it, if the program needs to process various sections of the document out of sequential order.
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Storing XML documents as large objects within a database is an excellent solution for some types of SQL/XML integration. If the XML documents are, for example, text-oriented business documents, or if they are text components of web pages, then there is really very little need for the DBMS to understand the internals of the XML documents themselves. Each document can probably be identified by one or more keywords or attributes, which can easily be extracted and stored as conventional columns for searching. If the XML documents to be processed are really data processing records, however, the simple integration provided by large objects may be too primitive. You will probably want to process and access individual elements, and search based on their contents and attributes. The DBMS already provides these capabilities for its native row/column
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data. Why can t the DBMS automatically decompose an incoming XML document, transforming its element contents and attribute values into a corresponding set of internal row/column data for processing On the outbound side, we have already seen how this approach can work to transform row/column query results into an XML document. The same technique could be used to recompose an XML document if it were once again needed in its external text form. The challenge of transforming XML documents, which are an excellent external data representation, to and from internal data representations more useful for programs is not unique to database systems. The same problems occur, for example, in Java processing of XML, where it is very desirable to transform an XML document to and from a set of Java class instances for internal processing. The process of decomposing an XML document into its component elements and attributes in some internal, binary representation is called unmarshalling in the XML literature. Conversely, the process of reassembling these individual element and attribute representations into a complete text XML document is called marshalling. For very simple XML documents, the marshalling and unmarshalling process is straightforward, and commercial DBMS products are moving to support it. Consider once again the simple purchase order document in Figure 25-3. Its elements map directly, one to one, onto individual columns of the ORDERS table. In the simplest case, the names of the elements (or attributes) will be identical to the names of the corresponding columns. The DBMS can receive an inbound XML document like the one in the figure, automatically turn its elements (or attributes, depending on the style used) into column values, using the element names (or attribute names) to drive the process. Reconstituting the XML document from a row of the table is also no problem at all. The DBMS must do slightly more work if the element names in the XML document don t precisely match column names. In this case, some kind of mapping between element names (or attribute names) and column names must be specified. It s relatively straightforward to put such a mapping into the DBMS system catalog. 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 unmarshalled 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 marshalling 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 marshalling/unmarshalling of even this simple document requires a more complex mapping.
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