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SQL/XML, XQuery , and Native XML Programming Languages

<root> <presentation> <author> William Beaumont </author> <presenter> William Beaumont </presenter> <based-on idref = " thePaper " /> </presentation> <paper id = " thePaper " > <title> SQL/XML, Xquery , and Native XML Programming Languages </title> <author> <name> Jonathan Robie </name>

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SQL/XML, XQuery , and Native XML Programming Languages

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  1. <root> <presentation> <author>William Beaumont</author> <presenter>William Beaumont</presenter> <based-onidref="thePaper"/> </presentation> <paperid="thePaper"> <title>SQL/XML, Xquery, and Native XML Programming Languages</title> <author> <name>Jonathan Robie</name> <company>DataDirect Technologies</company> </author> <yearPublished>2003</yearPublished> <publisher>IDEAlliance</publisher> <publishingVenue>XML Conference &#38; Exposition 2003</publishingVenue> <link> http://www.idealliance.org/papers/dx_xml03/papers/05-02-01/05-02-01.pdf </link> </paper> </root> SQL/XML, XQuery, and Native XML Programming Languages

  2. Written and presented by William Beaumont Based on the article byJonathan RobieofDataDirect Technologies, published 2003 by IDEAlliance at XML Conference & Exposition 2003 (http://www.idealliance.org/papers/dx_xml03/papers/05-02-01/05-02-01.pdf) SQL/XML, XQuery, and Native XML Programming Languages

  3. 1. Introduction • A web application usually uses XML to transfer data from itself to a relational database and vice versa • Every major database vendor uses its own proprietary approach to implementing this transference • There is no interoperability between the vendors’ approaches • Many developers need to make applications that work for databases from different vendors

  4. 1. Introduction (cont’d): XQuery and SQL/XML • XQuery and SQL/XML are separate standards for query languages that return queried data as XML • What both standards have in common: • The returned XML can have any desired structure • The queries can be arbitrarily complex • What differs between the standards: • XQuery is XML-centric • SQL/XML is SQL-centric

  5. 1. Introduction (cont’d): SQL/XML • SQL/XML is an SQL extension (specifically, it is part of ANSI/ISO SQL 2003) • It lets SQL queries create XML structures using several XML publishing functions • It’s easy to learn for an SQL programmer because it encompasses a small number of additions to the SQL language • Naturally, as an extension to SQL, it can be used in conjunction with any or all of SQL’s tools/infrastructure • For example, SQL/XML can be used with JDBC, SQL’s interface to Java • There is no equivalent standard API for XQuery (or at least there was none at the time of this article’s writing in 2003) • Also, SQL, being a mature language, has functionality not yet available to XQuery, such as updates or stored procedures

  6. 1. Introduction (cont’d): XQuery • XQuery uses XML as the basis for its data model and type system • It is currently under development by the World Wide Web Consortium (W3C) • It can be described as a Native XML Programming Language • XML:XQuery :: relational model:SQL • Many products and projects offer ways to query relational data using an XML view of the database • XQuery’s design was influenced by this relational-data-as-XML paradigm • Ultimately, XQuery lets you work in the XML world no matter what type of data you’re working with: XML, relational, object, and so on • XQuery and SQL/XML are more or less equivalent when it comes to queries based purely on relational data • XQuery has the advantage with queries that span relational and XML sources

  7. 2. XML and Relational • XML and relational databases are based on two very different data models • Relational model: • 2-dimensional tables • No hierarchy or significant order within tables • XML: • Trees • Order is significant • Hierarchy and sequence are the main ways to represent information

  8. 2. XML and Relational: Representation • XML’s hierarchical, ordered data model is more naturally suited to presentation than the relational model • Suppose we had the following tables in a relational database: Projects Customers If we wished to query all the information about each customer, including their associated projects, we would use the following query: • SELECT * FROM Customers c, Projects p • WHERE c.CustId=p.CustId • ORDER BY c.CustId, p.ProjId

  9. 2. XML and Relational: Representation (cont’d) • The result of this query would be: • Note the rows with duplicate information, which have been highlighted

  10. 2. XML and Relational: Representation (cont’d) • By contrast, the same information can be modeled in XML as follows: <customers> <customer id="1"> <name>Woodworks</name> <city>Baltimore</city> <projects> <project><name>Medusa</name></project> </projects> </customer> <customer id="4"> <name>Hardware Shop</name> <city>Washington</city> <projects> <project><name>Pegasus</name></project> <project><name>Typhon</name></project> </projects> </customer> <!--...--> </customers>

  11. 3. XML and Relational: Four Approaches • XML applications that use relational data can choose from four approaches: • Use an interface like JDBC together with a parser like SAX or DOM, and maybe even XSLT, to transform SQL results into XML • What if the desired information is spread across multiple tables? • Requires lots of tedious code • Use the XML extensions provided by database vendors • These extensions are proprietary • What if we want a database-independent solution? • Use SQL/XML • Requires little new learning for an experienced SQL programmer • Is supported by Oracle and IBM • Is not supported by Microsoft • Database-independent implementations are available for any major relational DB

  12. 3. XML and Relational: Four Approaches (cont’d) • Use XQuery • Native XML query language • Natural to learn for XML programmers • Very good for applications that process XML together with relational data • Support for XQuery among major vendors is limited • There is not yet a complete standardized API

  13. 4.1 SQL/XML: XML Publishing Functions

  14. 4.1 SQL/XML: XML Publishing Functions (cont’d) • Example: select xmlelement(name "CustomerProj", xmlforest(c.CustId, c.Name as CustName, p.ProjId, p.Name as ProjName)) from Customers c, Projects P where p.CustId=c.CustId order by c.CustId select * from Customers c, Projects p where c.CustId=p.CustId order by CustId, p.projId <CustomerProj> <CustId>1</CustId> <CustName>Woodworks</CustName> <ProjId>1</ProjId> <ProjName>Medusa</ProjName> </CustomerProj> <CustomerProj> <CustId>4</CustId> <CustName>Hardware Shop</CustName> <ProjId>2</ProjId> <ProjName>Pegasus</ProjName> </CustomerProj> ...

  15. 4.2 SQL/XML: The XML Datatype • The XML datatype is a datatype used in SQL in the same way we use integer or date • The output of XML publishing functions goes in one or more columns of the result set just like any other SQL function • These columns are tagged as having the XML datatype for the benefit of any applications that use the result set • For example, the query "select Name as CustomerName, xmlelement(name CustomerCity, City) as CustomerCity" would return this result set:

  16. 4.2 SQL/XML: The XML Datatype (cont'd) • This is the Java code that would use the result set and expect the XML datatype: import com.ddtek.jdbc.jxtr.XMLType; import org.w3c.dom.*; public class MyClass { // ... public void myMethod() { // ... Statement statement = connection.createStatement(); ResultSetresultSet = statement.executeQuery(query); while(resultSet.next()) { String customerName = resultSet.getString(1); XMLTypecustomerCityXML = (XMLType)resultSet.getObject(2); Document doc = customerCityXML.getDOM(); doSomethingUseful(customerName, doc); } // ... } // ... }

  17. 4.3 SQL/XML: Mapping Rules • XML publishing functions use SQL values to create XML values whose types come from W3C XML Schema • SQL/XML mapping rules describe how SQL values are mapped to and from XML values • They also describe how SQL metadata is mapped to and from W3C XML Schemas

  18. 4.3 SQL/XML: Mapping Rules (cont'd) • An example of a decision in SQL/XML mapping is whether to map a table to one element or to a forest of elements, with one tree for each row <Table> <row> <A>Value 1A</A> <B>Value 1B</B> <C>Value 1C</C> </row> <row> <A>Value 2A</A> <B>Value 2B</B> <C>Value 2C</C> </row> </Table> <Table> <A>Value 1A</A> <B>Value 1B</B> <C>Value 1C</C> </Table> <Table> <A>Value 2A</A> <B>Value 2B</B> <C>Value 2C</C> </Table>

  19. 4.3 SQL/XML: Mapping Rules (cont'd) • Mappings are also defined on the metadata level • SQL/XML defines how SQL datatypes are represented in XML Schema • Each SQL type is derived from an equivalent built-in W3C XML Schema type

  20. 4.3 SQL/XML: Mapping Rules (cont'd) Customer(id INTEGER, name CHAR(50)) <xsd:schemaxmlns:xsd="http://www.w3.org/2001/XMLSchema"> ... <xsd:simpleTypename="INTEGER"> <xsd:restrictionbase="xsd:int" /> </xsd:simpleType> <xsd:simpleTypename="CHAR_50"> <xsd:restrictionbase="xsd:string"> <xsd:lengthvalue="50" /> </xsd:restriction> </xsd:simpleType> ... <xsd:elementname="Customer"> <xsd:complexType> <xsd:sequence> <xsd:elementname="id" type="INTEGER" /> <xsd:elementname="name" type="CHAR_50" /> </xsd:sequence> </xsd:complexType> </xsd:element> ... </xsd:schema>

  21. 5. XQuery and Native XML Programming • XQuery takes sequences of XML nodes as input and returns a sequence of XML nodes • It also allows XML views of non-XML data and serialized forms of non-XML data • We can store, query, process, and exchange data as XML using XQuery • Conventional Web applications only exchange data as XML and use SQL for storing and querying and Java or C# for processing • This can result in mismatches

  22. 5.1 XQuery: Native XML Programming • Everything that is queried using XQuery must be processed as though it were XML • We can either serialize the data as XML or create an XML view of it • Most systems that use relational data use SQL/XML mappings to create XML views • XQuery's fundamental types: • Document nodes • Elements • Attributes • Processing instructions • Comments • Text nodes • W3C XML Schema types (integers, strings, dates, etc.) • XQuery doesn't require schemas, but a schema can be specified if a query must ensure consistent use of types • XML is the basis of XQuery's type system and data model, and so, like XSLT and XPath, it is considered a Native XML Programming Language • SQL is the basis of SQL/XML, and so it is not considered a Native XML Programming Language • SQL/XML is merely a bridge to XML

  23. 5.1 XQuery: Native XML Programming (cont'd) • XML is not Objects! • An XML parser like DOM treats XML as though objects were its fundamental type • This leads to messy code like this: Tree t = ParseXML("stock.xml"); PERatio = number(t.getmember("/stock/price")) / ((number(t.getmember("/stock/revenues") - number(t.getmember("/stock/expenses")) • The same code can be done much more simply in XQuery like this: let $stock := document('stock.xml')/stock return $stock/price div ($stock/revenue - $stock/expenses)

  24. 5.1 XQuery: Native XML Programming (cont'd) • XML is not just text! • The XML documents below are not the same textually, but logically they are identical: <item xmlns:dc="http://purl.org/dc/elements/1.1/"> <title>MetaData</title> <dc:date>2003-01-12T00:18:05-05:00</dc:date> <link>http://bitworking.org/news/8</link> <description>Upon waking, the dinosaur...</description> </item> <root:itemxmlns:bc="http://purl.org/dc/elements/1.1/" xmlns:root=""> <root:title>MetaData</root:title> <bc:date>2003-01-12T00:18:05-05:00</bc:date> <root:link>http://bitworking.org/news/8</root:link> <root:description>Upon waking, the dinosaur...</root:description> </root:item> • Using regular expressions to process the text of XML documents can thus be a programmer's nightmare

  25. 5.1 XQuery: What should a Native XML Programming Language do? • Easily find anything in an XML structure (i.e., XPath) • Easily create any XML structure • Easily combine and restructure information from XML sources • Example: for $c in $cut/row let $p := $proj/row[CustId = $c/CustId] return <customer> <custName>{ string($c/name) }</custName> <projName>{ string($p/name) }</projName> </customer> • Easily use XML data in expressions • E.g., we should be able to apply arithmetic operations directly to XML content, observing the data types of typed data • Schemas that have been applied to a query should have their type constraints enforced

  26. 5.2 XQuery and SQL/XML Views • XQuery has few advantages over SQL/XML in terms of creating XML structures from relational data • Below are the SQL/XML and XQuery implementations of a relational data query whose output is XML: select xmlelement(namecustomer, xmlattributes(c.CustIdasid), xmlforest(c.Nameasname, c.Cityascity), xmlelement(nameprojects, (selectxmlagg(xmlelement(nameproject, xmlattributes(p.ProjIdasid), xmlforest(p.Nameasname))) fromProjectsp wherep.CustId=c.CustId))) as "customer-projects" fromCustomersc for $c in $cust/row return <customer id="{$c/CustId}"> <name>{string($c/Name)}</name> <projects> { for $p in $proj/row where $p/CustId=$c/CustId return <project id="{$p/ProjId}" name="{$p/Name}" /> } </projects> </customer>

  27. 5.3 Spanning Sources: XQuery, Web Messages, and Databases • Since XQuery works well on SQL/XML views of relational data, its strength is in processing both XML and relational data • Suppose we need to process a request in the form of an XML file by querying a relational database and returning an XML response • Below is the XML request: <p:itinerary xmlns:p="http://travel.org/reservation/travel"> <p:departure> <p:departing>New York</p:departing> <p:arriving>Los Angeles</p:arriving> <p:departureDate>2001-12-14</p:departureDate> <p:departureTime>late afternoon</p:departureTime> <p:seatPreference>aisle</p:seatPreference> </p:departure> </pi:itinerary>

  28. 5.3 Spanning Sources (cont'd) Original request • Below is the XQuery response, which looks up airport and flight information in a relational database: for $city in doc("incoming.xml")//p:departing let $airports := sql:table("airports")/AIRPORTS/row[CITY = $city] return if(count($airports) = 0) then <error>No airports found for {$city}!</error> else if(count($airports) = 1) then <airport>{string($airports/AIRPORT)}</airport> else if(count($airports) > 1) then <airportChoices> { for $c in $airports/AIRPORT return (string-value($c), " ") } </airportChoices> else() Relational DB

  29. 6. SQL/XML and XQuery: Are both needed? • SQL/XML is best for SQL programmers who think of their tasks in SQL terms and need to create results in XML • SQL/XML is like an SQL reporting tool, except that it creates reports in XML • XQuery is best for XML programmers who are working either with only XML data or with both XML and relational data • When XQuery needs to work with relational data, it needs an SQL/XML view of the relational data source; in this way, XQuery and SQL/XML work well together • SQL/XML works well in traditional SQL environments, provides full access to the standard SQL language, has APIs like JDBC and ODBC, and has implementations provided by Oracle and IBM • XQuery is a young language, with currently no standardized support for updates and full-text search and no complete API • The bottom line is that if you're working with only XML data as your input(s), use XQuery • If you're working with only relational data as your input(s), use SQL/XML • If you're working with both types of data as your input(s), use XQuery with SQL/XML views

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