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2.3. XML QUERY LANGUAGES 31 m. p consists of two leading /a/b location steps followed by m − 1 expressions of the form /../b. The first part selects the two b elements. The remainder of p is executed for both b. Each /../b step returns the two b elements, so that the number of b elements is doubled on each execution of the /../b path expression. The m-th execution step returns 2 m b elements. Of course these are redundant repetitions of the original two b elements. This naive approach is implemented in most XPath processors like XALAN [7] and Microsoft Internet Explorer 1 Gottlob, Koch and Pichler propose a more efficient implementation that uses polynomial runtime in the worst case. Details can be taken from [34, 35]. 2.3.4 XQuery XQuery is a functional programming language that uses XPath expressions for selecting nodes. More precisely, XQuery is a superset of XPath; this implies that each XPath expression is already a valid XQuery expression and will return the same result in both languages. In contrast to XPath that only selects nodes in a given document XQuery allows to transform the nodes and to create new structures with XML templates. XQuery is a proposal of the W3C that is not standardized yet. The latest working draft was published in April 2005. However, many systems that use an XQuery implementation are realized. Examples include the Microsoft SQL Server 2005 Express with XQuery support [79] or the Berkeley DB XML 2.0 [107], an embedded native XML database with support for XQuery 1.0. Sequences The basic construct of XQuery are sequences. All XQuery expressions and functions operate on one or multiple sequences and returns a new sequence. Sequences consist of items which can be atomic values or XML nodes. Both types can be mixed in a sequence. A sequence may not contain another sequence but can contain nested nodes. The following example shows how a sequence of values and elements is created syntactically in XQuery: 1 (1 , , 2 , 1 , ) The items in a sequence are ordered and may appear more than once. Therefore, duplicates are allowed in a sequence. The concept of the sequence is different to the node-sets of XPath that have no ordered items and no duplicates. 1 The Internet Explorer uses XPath in its embedded XSLT engine generating HTML pages from an XML document and a stylesheet.
32 CHAPTER 2. FUNDAMENTALS FLWOR-Expressions In contrast to XPath that only selects existing nodes, XQuery allows the construction of XML results that have a different structure than the original data. The central framework of XQuery are the FLWOR-expressions. The acronym FLOWR stands for for-let-where-order by-return and is pronounced like the word ’flower’. FLWOR-expressions support the binding on variables, the iteration over sequences and templates that define the structure of the resulting XML data. This way FLWOR can be compared to SQL in relational databases. A FLWOR-expression creates a sequence of nodes and values by binding the results of a path expression to a variable by the use of for and let . The items of the sequence are filtered by the conditions expressed in the where clause. Optionally, the remaining items can be ordered with order by and returned with the return statement. This process is illustrated in figure 2.11. Figure 2.11: Construction of a FLWOR-Expression The let and for clause bind the results of an XQuery expression (the result is a sequence) to a specific variable that can be addressed by further processing steps. The semantic of let and for is different: The let clause binds the sequence as a whole to the variable. Therefore, the further processing steps are performed once for the sequence. The for clause binds all items of the sequence to the variable so that the further processing is performed with each item separately. The different semantic of let and for is illustrated in a small example: The first XQuery expression binds a sequence of three element-nodes to the variable x. The return statement creates an item element and encloses the content of x. 1 l e t $x : = (, , ) 2 return { $x } The resulting XML data is as follows: 1 2 3 4 5
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170 BIBLIOGRAPHY [12] Alberto Capra
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174 BIBLIOGRAPHY [59] Raghav Kaushi
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176 BIBLIOGRAPHY [84] David G. Mitc
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178 BIBLIOGRAPHY [113] W3 Schools.
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182 INDEX Oracle XML DB, 41 Parent,
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