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Michael M. David returns to SQLSummit.com to explore two approaches to XML hierarchical structure transformations. He explains the difference between restructuring and reshaping XML data, with the latter driven by the semantics of the data structure.
Today the structure transformation terminology of Restructuring and Reshaping are used interchangeably for XML structure transformation processes. There are two basic types of XML hierarchical structure transformations that need to be distinguished because they are different in meaning, results, and use. These are Restructuring controlled by existing relationships in the data, and Reshaping controlled by the semantics of the current data structure. Restructuring is performed by using new and unused relationships to restructure the data. On the other hand, Reshaping uses the structure semantics of the current structure to mold the structure into any other shape without requiring or relying on any relationships in the data. Restructuring using data relationships can create a new structure and data with new semantics, while reshaping using structure semantics alters the structure without changing the data and its semantics. Both have their use. Restructuring is usually used to match a structure to its application use, and Reshaping to map a structure to a desired structure format.
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Restructuring and Reshaping can both be performed or conceptually specified by using join conditions on multiple physical or logical copies of the same structure. Restructuring uses data relationships while Reshaping uses the same data field in the structure copies to synchronize them. Reshaping is also used in inverting structures when data relationships are not available in the data. This is often the case with XML data since foreign keys are not required because of their contiguous nested storage. This technique has been adapted for use in different database processors. XQuery procedural nested looping in its FLOWR statement can navigate linear and nonlinear structures, and along with its join capability, perform both Restructuring and Reshaping of linear and nonlinear structures using the structure transformation techniques described here.
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