Publication: XML Journal
Publication Date: 01-DEC-03
Delivery: Immediate Online Access
Author: David, Michael M
Article Excerpt
Part 1 of this article demonstrated how standard ANSI SQL can integrate fully, naturally, and seamlessly with XML. This was accomplished by naturally raising SQL processing to a hierarchical level, enabling relational data (including XML-shredded data) to integrate at a full hierarchical level with native XML. Hierarchical processing and the utilization of the hierarchical semantics were also shown in Part 1, along with the hierarchical joining of hierarchical structures.
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Part 2 will cover how standard SQL can naturally support more advanced XML capabilities such as made promotion, fragment processing, structure transformation, variable structures, and the handling of shared and duplicate elements.
Node Promotion and Fragment Processing
SQL's hierarchical processing capabilities do not stop with what was presented in Part 1. We will look at supporting XML's more advanced capabilities starting with node promotion and fragment processing. Node promotion occurs when a node in a hierarchical structure moves up past its parent and ascendants that have not been selected for output (this is controlled by projection in relational terms). This slicing out of nodes in the structure definition has the same effect as slicing them out when the data selected is transferred from the relational Working Set to the Result Set. This is shown at the bottom of Figure 1. This means that this standard XML hierarchical processing capability is also performed naturally in relational processing when a node (table or element) is not selected for output.
[FIGURE 1 OMITTED]
From what we saw in Part 1, the basic operation of SQL is to use the SELECT clause to specify the desired output data; the FROM clause to specify its input data; the LEFT JOIN to specify its data structure; and the WHERE clause to specify optional data filtering criteria. Using these simple and intuitive SQL language constructs, even more complex node promotion leading to fragment processing can be easily performed. The query in Figure 1 joins a structure fragment selected from the lower structure to the upper structure. The structure fragment is shown encircled in a dashed oval. Dashed boxes represent nodes that are not selected for output.
A fragment is a portion or grouping of nodes from a hierarchical structure that retains its basic hierarchical structure when unselected nodes are removed, enabling it to be manipulated as a unified structure. It can be embedded below the original root of the input structure and can be a loose collection of nodes like the structure fragment shown in Figure 1. It is defined by the nodes that ate selected for output from its defined input structure. In this example, the fragment is located below the root node D of its original input structure, which contains nodes O and I, which are not selected for output and will not be included in the fragment. This causes nodes Y and J to be automatically promoted over nodes O and I so they both naturally collect under the fragment root node F. In fact, the whole process of fragment creation and processing is made possible by the natural action of node promotion.
With the structure fragment identified and isolated by data selection, it can be naturally joined to the upper structure as shown in Figure 1. When linking to a lower-level fragment, it is usually to its root node as in this example, where...
NOTE: All illustrations and photos have been removed from this article.
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