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Unit III

XML Databases

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XML Querying XML Query Languages:

XPath Provides language constructs that specifies path

expressions. These path expressions identifies nodes (elements) in an

XML document.

These nodes match the patterns specified in the pathexpression.

XQuery More general query language

XQuery uses XPath expressions

Has some additional constructs

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XML Querying XPath

An XPath expression returns a collection of element nodesthat satisfycertain patterns specified in the expression.

The names in the Xpath expression are node names in theXML document tree.

They are either tag (element) names or attribute nameswith additional qualifier conditions.

Qualifier conditions further restrict the nodes that satisfythe pattern.

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XML Querying XPath

There are two main separators when specifying a path. Single Slash (/) and Double Slash (//)

A single slash before a tag specifies that the tag mustappear as the direct child of the previous parent tag.

A double slash specifies that the tag can appear as adescendent of the previous tag at any level.

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XML Querying XPath

It is customary to include the file name in any XPath queryallowing us to specify any local file name or path name that

specifies the path. doc(www.company.com/info.XML)/company => Company

XML doc

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XML Querying Examples of XPath Expressions on XML documents

that follow the XML schema file COMPANY

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XML Querying

1. /company

Returns the COMPANY root node and all its descendent nodes.

It returns the whole XML document. 2. /company/department

Returns all the department nodes (elements) and theirdescendent subtrees.

3. //employee [employeeSalary gt 70000]/employeeName

Returns all employeeName nodes that are direct children of theemployee node, such that employeeSalary (another childelement of employee node) value is greater than 70000.

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XML Querying

4./company/employee[employeeSalary gt 70000]/employeeName

Returns the same result as above one.

Only difference is that full path name is specified

5. /company/project/projectWorker [hours ge 20.0]

Returns all projectWorker nodes and their descendent nodesthat are children under the path /company/project that have achild node hours with value greater than or equal to 20.0 hours.

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XML Querying XQuery

XQuery uses XPath expressions but has additionalconstructs.

XQuery permits the specification of more general querieson one or more XML documents.

The typical form of query in XQuery is known as FLWRexpression, which stands for the four main clauses ofXQuery and has the following form: FOR

LET

WHERE

RETURN

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XML Querying

Examples

ofXquery

queries

on

XML

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XML Querying

XQuery

1. First query retrieves the first and last names ofemployees who earn more than 70000. Variable $x is bound to each employeeName element child

of an employee element only if the employee lementsatisfies the qualifier that employeeSalary is greater than70000.

2. Second query also returns the same result as the firstquery. This is another way of representing the same query

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XML Querying

XQuery

3. Third query illustrates how a join operation can beperformed by having more than one variable. $x variable is bound to each projectWorker element that is

the child of project number 5.

$y variable is bound to each employee element

The join condition matches the SSN values in order to

retrieve the employee names.