Databases get a grip on XML
If you could do one thing to improve integration and automate processes with customers and business partners, it would be to implement XML, which has become the standard for exchanging information between disparate systems because it is easily transformed into any format. The good news is that the four leading relational databases, namely Oracle Database, IBM DB2, Sybase ASE, and Microsoft SQL Server, not only can store XML data, but they hide much of the complexity of working with XML.
What does a fashionable XML database provide? Four basic functions: the ability to consume, store, search, and generate XML. The extent to which the database supports these functions and the methods it uses to accomplish them are what make for a successful implementation of XML in a database.
Relational databases and XML documents are both powerful ways to represent relationships among data, but they're powerful in different ways. For example, querying on a patient ID number in a relational database may allow you to quickly find the dates a certain patient visited the hospital, the conditions he was diagnosed with, and the treatments he was given. But it likely won't help you determine which treatments were provided for which conditions or what times the treatments took place, nor will it give you other useful information that XML versions of these records could provide.
But whether or not you can combine the benefits of relational and XML data depends on how you store the XML. There are three methods for physically storing XML data in a relational database: shredded, unstructured, and structured. Shredded and unstructured are useful methods but limited. The structured method allows you to leverage the power of both relational data and XML hierarchies.
Shredding puts XML data into relational columns but strips it of its XMLness, meaning the hierarchical relationships among the data in the original XML document are lost. Shredding is useful when you're not concerned about keeping the data in XML format. For example, let's say you have a Web site that allows customers to place orders, and the order needs to go to a number of different database systems. Producing an XML file and having the different systems pick it up——that is, shred it——from a network share may be the most efficient and error-free way to get the data where you want it to go.
The unstructured method uses a data type called a CLOB (Character Large Object) to store an entire XML document as a single unit. Databases have been doing this for years with different types of documents, so this is nothing new. The unstructured method provides limited search capabilities, but it is still quite useful. You can't base queries on it, but the structure of the original data is preserved. A good use for unstructured XML storage would be in keeping original documents to comply with government regulations. For example, if a financial institution were to receive original loan documents in XML, this would allow them to have a relational record of each loan application, and also to store the original application with that record.
The structured method allows you to store XML data inside the database and preserve the hierarchy of the data. Structured storage, also known as “native XML”storage, is what every vendor is trying to achieve. The most obvious benefit of preserving the hierarchical relationships of XML data is being able to receive an XML document, combine it or manipulate it with relational data, and produce XML as a result. It isn't possible to produce such result sets with a relational query language alone.
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2011/2/21 10:28:00
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Databases get a grip on XML(3711字) － hjx_221，2011年2月21日
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