Querying XML Data Based on Nested Relational Sequence Model
Ho Lam Lau, Wilfred Ng · 2002
We propose the Nested Relational Sequence Model (NRSM) [6], which is an extension of the well-established Nested Relational Data Model (NRDM) [2,3,4] in order to cater for the two important features of nesting structure and node ordering in XML documents [1,5]. The NRSM supports composite and multi-valued attributes, which are essential for representing hierarchically structured objects such as XML data. In addition, the NRSM extends the NRDM to support ordering of XML data by allowing nested tuple sequences in a nested sequence relation (or a NRS relation). An important feature in our model is that XML data that has the same label along the same path can be collapsed into the same data node. This eliminates a substantial amount of redundancy in an XML document. A NRS relation R is defined by R = (N, O, S), where N is the NRS name, O is the NRS occurrence and S is the NRS schema. In Figure 1 we show an example of mapping of an XML data tree into a NRS relation. Within the NRSM we define a set of algebraic operations, which is employed to formulate a query over a NRS relation that contains XML data. These operations enable users to retrieve XML information and to integrate XML documents in a systematic manner as follows: by taking one or more NRS relations as an input, a NRS query returns a NRS relation as an output result which represents an XML data tree. We summarize these operations in Table 1 given below.