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To eliminate redundancies in XML documents, some normal forms (XNF) for XML schemas have been proposed [1, 24, 4, 5].
However, [S.sub.2] is not in XNF, since its instance [I.sub.2] does not satisfy the key
However, neither [S.sub.2] nor [S.sub.3] is in XNF. We have already shown that there is redundancy in instances of [S.sub.2].
In Figure 9 there is schema [S.sub.4] that is in XNF. To make the example more illustrative, we added node name to part data.
It can be shown that [S.sub.4] satisfies the condition of XNF. Thus, this schema is both redundant-free and dependency preserving.
In the previous section we discussed an example of transforming an XML schema into XNF. We started with the schema [S.sub.1] in Figure l, and the final schema was [S.sub.4] in Figure 9.
Starburst's eXtended Normal Form (XNF) is based upon the concept of a relational view.
An XNF query is identified by the keywords OUT OF and consists of : 1) definitions of components tables (called 'nodes'), identified by the keyword SELECT; and 2) definitions of directed relationships (called 'edges') between component nodes (identified by the keywords RELATE <nodes> WHERE <predicates>.
XNF objects may be combined, proejcted, and restricted.
To process XNF queries, Starburst's extensible query processor also had to be extended.
Recently, we have implemented techniques for updating XNF views, including those containing joins [55]; similar techniques were independently developed in [9].