XML –Query Languages, Extracting from Relational Databases

ADVANCED DATABASES

Khawaja MohiuddinAssistant Professor

Department of Computer SciencesBahria University (Karachi Campus)

Khawaja.mohiuddin@bimcs.edu.pkhttps://sites.google.com/site/khawajamcs

Content for this lecture is taken from: Chapter 11 of “Database Systems: Models, Languages …”, 6th Ed.” by Elmasri and Navathe(Chapter 12 of “Fundamentals of Database Systems” 6th Ed. by Elmasri and Navathe)

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Topics to Cover

XML Languages Xpath XQuery

Extracting XML Documents from Relational Databases

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XML Languages

Two query language standards emerged XPath

• Provides language construct to specify path expressions to identify certain nodes (elements) or attributes within an XML document that match specific patterns

XQuery• Uses XPath expressions but has additional

constructs

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XPath: Specifying Path Expressions in XML

XPath expression Returns a sequence of items that satisfy a

certain pattern as specified by the expression Returned items are either values (from leaf

nodes) or elements or attributes Qualifier conditions

• Further restrict nodes that satisfy pattern Separators used when specifying a path:

Single slash (/) - specifies that the tag must appear as a direct child of the previous (parent) tag

Double slash (//) - specifies that the tag can appear as a descendant of the previous tag at any level

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XPath: Specifying Path Expressions in XML (cont’d.)

1. Returns the company root node and all its descendant nodes, i.e. returns the whole XML document.

2. Returns all department nodes (elements) and their descendant subtrees.

3. // is convenient if we do not know the full path name we are searching for, but do know the name of some tags of interest within the XML document. Returns all employeeName nodes that are direct children of an employee node, such that the employee node has employeeSalary value greater than 70000.

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XPath: Specifying Path Expressions in XML (cont’d.)

XPath has a number of comparison operations for use in qualifier conditions, including standard arithmetic, string, and set comparison operations.

4. Returns the same result as the previous one, except that the full path name is specified in this example.

5. Returns all projectWorker nodes and their descendant nodes that are children under a path /company/project and have a child node hours with a value greater than or equal to 20.0 hours.

Note: For COMPANY XML document, stored at www.company.com/info.xml, the first XPath expression should be written as:

doc(www.company.com/info.xml)/company

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XPath: Specifying Path Expressions in XML (cont’d.) To include attributes in Xpath expression,

attribute name is prefixed by the @ symbol.

Wildcard symbol * Stands for any element Example: /company/* the result can be a sequence of different

types of items

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XPath: Specifying Path Expressions in XML (cont’d.) Axes

Our previous examples only included three: 1. Child of current node (/)2. Descendent or self at any level of current not

(//)3. Attribute of the current node (@)

A more general model for path expressions called Axes has been proposed.

In it, it is possible to move in multiple directions from current node in path expression

Includes self, child, descendent, attribute, parent, ancestor, previous sibling, and next sibling

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XPath: Specifying Path Expressions in XML (cont’d.)

Main restriction of XPath path expressions Path that specifies the pattern also specifies

the items to be retrieved Difficult to specify certain conditions on the

pattern while separately specifying which result items should be retrieved.

The XQuery language separates these two concerns, and provides more powerful constructs for specifying queries

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XQuery: Specifying Queries in XML

The typical form a query in XQuery is known as FLWR expression FLWR stands for the four main clauses of XQuery

1. FOR <variable bindings to individual nodes (elements)>

2. LET <variable bindings to collections of (elements)>

3. WHERE <qualifier conditions>4. RETURN <query result specification>

Zero or more instances of FOR and LET clauses allowed

WHERE clause is optional, but can appear at most once

RETURN clause must appear exactly once

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1. Variables are prefixed with the $ sign.

2. LET clause assigns a variable to a particular expression for the rest of the query.

3. FOR clause assigns a variable to range over each of the individual items in a sequence.

4. WHERE clause specifies additional conditions on the selection of items.

5. RETURN clause specifies which elements or attributes should be retrieved from the items that satisfy the query conditions.

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Other Examples:

XQuery: Specifying Queries in XML (cont’d.)

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XQuery: Specifying Queries in XML (cont’d.) XQuery contains powerful constructs to

specify complex queries: Conditional quantifiers, aggregate

functions, ordering of query results, selection based on position in a sequence, and even conditional branching. Hence, in some ways, it qualifies as a full-fledged programming language

www.w3.org Contains documents describing the latest

standards related to XML and XQuery

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Other Languages and Protocols Related to XML The goal is to provide the technology for

realization of the Semantic Web, where all information in the Web can be intelligently located and processed.

Extensible Stylesheet Language (XSL) Define how a document should be rendered

for display by a Web browser Extensible Stylesheet Language for

Transformations (XSLT) Transform one structure into different

structure

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Other Languages and Protocols Related to XML (cont’d.)

Web Services Description Language (WSDL) Allows description of Web Services in XML

making the Web Service available to users and programs over the Web.

Simple Object Access Protocol (SOAP) Platform-independent and programming

language-independent protocol for messaging and remote procedure calls

Resource Description Framework (RDF) Provides languages and tools for exchanging

and processing of meta-data (schema) descriptions and specifications over the Web

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Extracting XML Documents from Relational Databases Creating hierarchical XML views over flat

or graph-based data Representational issues arise when

converting data from a relational database system into XML documents

UNIVERSITY database example

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In this hierarchy, the combined COURSE/SECTION information is replicated under each student who completed the section.

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Breaking Cycles to Convert Graphs into Trees Complex subset with one or more cycles

Indicate multiple relationships among the entities

Even more difficult to decide how to create the document hierarchies

Can replicate the entity types involved to break the cycles and convert into a hierarchy

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Breaking Cycles to Convert Graphs into Trees (cont’d.)

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Other Steps for Extracting XML Documents from Databases1. Create correct query in SQL to extract

desired information for XML document2. Restructure query result from flat

relational form to XML tree structure3. Customize query to select either a

single object or multiple objects into document. For example: In Figure 12.13, the query can select a single student entity and create a document corresponding to that single student, or it may select several, or even all of the students and create a document with multiple students.

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Conclusion

XPath and XQuery languages Query XML data

Extracting XML documents from relational databases involve converting from flat to hierarchical structure.

There could be cycles which need to be broken by replicating some entities.

Other than conversion some additional steps need to be taken

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