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9 Querying XML Data and Documents

9 Querying XML Data and Documents. XQuery , W3C XML Query Language "work in progress", Working Draft, 30 April 2002 joint work by XML Query and XSL Working Groups together with XPath 2.0 ( <- XPath 1.0 and XQuery) influenced by many research groups and query languages

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9 Querying XML Data and Documents

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  1. 9 Querying XML Data and Documents • XQuery, W3C XML Query Language • "work in progress", Working Draft, 30 April 2002 • joint work by XML Query and XSL Working Groups • together with XPath 2.0 (<- XPath 1.0 and XQuery) • influenced by many research groups and query languages • Quilt, XPath, XQL, XML-QL, SQL, OQL, Lorel, ... • Goal: a query language applicable to any XML-represented data: both documents and databases Notes 9: XQuery

  2. Functional Requirements (1) • Support operations (selection, projection, aggregation, sorting, etc.) on all data types: • Choose parts of data based on content or structure • Also operations on document hierarchy and order • Structural preservation and transformation: • Preserve the relative hierarchy and sequence of input document structures in the query results • Transform XML structures and create new XML structures • Combining and joining: • Combine related information from different parts of a given document or from multiple documents Notes 9: XQuery

  3. Functional Requirements (2) • Cross-references: • must be able to traverse intra- and inter-document references • Closure property: • The result of an XML query is also XML (usually not a valid document, but a well-formed document fragment) • The results of a query can be used as input to another query • Extensibility: • The query language should support the use of externally defined functions on all data types of the data model Notes 9: XQuery

  4. XQuery in a Nutshell • Functional expression language • Strongly-typed: (XML Schema) types may be assigned to expressions statically • Includes XPath 2.0 (says Draft, but not all XPath axes included!) • XQuery 1.0 and XPath 2.0 share extensive functionality: • XQuery 1.0 and XPath 2.0 Functions and Operators, WD 30/4/2002 • Roughly: XQuery  XPath' + XSLT' + SQL' Notes 9: XQuery

  5. XQuery: Basics • A query is represented as an expression • Expressions operate on and return sequences of • atomic values (of XML Schema simple types) and • nodes (same 7 node types as in XPath/XSLT) • an item  a singleton sequence • sequences are flat: no sequences as items • (1, (2, 3), (), 1) = (1, 2, 3, 1) • sequences are ordered, and can contain duplicates • Unlimited combination of expressions, often with automatic type conversions (e.g. for arithmetics) Notes 9: XQuery

  6. XQuery: Basics • Query applied to an input sequence,accessed through functions ... • input() • implementation-dependent input sequence • collection("URI") • sequence of nodes from URI • document("URI") • root of the XML document available at URI • this also in XSLT Notes 9: XQuery

  7. Some Central XQuery Expressions • path expressions • creation and combination of sequences (union, intersect, except) • element constructors ( XSLT templates) • FLWR (”flower”; for-let-where-return) expressions • sort expressions • quantified expressions (some/every … satisfies …) • testing and modifying datatypes; validation Notes 9: XQuery

  8. Path Expressions • Extend the syntax of XPath: [/]Expr/… /Expr • arbitrary (node-sequence-valued) expressions as steps • only 6 (of 13) axes: child, descendant, attribute, self, descendant-or-self, parent • combined with order comparison operators (precedes, follows) sufficient for queries (?) • produce ordered sequence of nodes in document order, without duplicate nodes • dereferencing for ID/IDREF values: Figure(s) referenced by refid attribute of the first figref child:figref[1]/@refid => figure Notes 9: XQuery

  9. Element Constructors • Similar to XSLT templates: • start and end tag enclosing the content • literal fragments written directly, expressions enclosed in braces { and } • often used inside another expression that binds variables used in the element constructor • See next Notes 9: XQuery

  10. Example • Generate an emp element containing an empid attribute and nested name and job elements; values of the attribute and nested elements given in variables $id, $n, and $j: <emp empid="{$id}"> <name>{$n} </name> <job> {$j} </job> </emp> Notes 9: XQuery

  11. FLWR ("flower") Expressions • Constructed from for, let, where and return clauses (~SQL select-from-where) • Form: (ForClause | LetClause)+ WhereClause? "return" Expr • a FLWR expression binds values to one or more variables, and uses these variables to construct a result (in general, an ordered sequence of nodes) Notes 9: XQuery

  12. Flow of data in a FLWR expression Notes 9: XQuery

  13. for clauses • for $V1inExp1 (, …) associates each variable Vi with an expression Expi that returns a list of items (e.g. a path expression) • Result: list of tuples, each containing a binding for each of the variables • can be though of as loops iterating over the items returned by respective expressions Notes 9: XQuery

  14. let clauses • let also binds one or more variables to one or more expressions • each variable to the entire value of its expression (without iterating through items of the sequence) • results in binding a single sequence for each variable • Compare: • for $x in /library/book-> many bindings (books) • let $x := /library/book-> single binding (to a sequence of books) Notes 9: XQuery

  15. for/let clauses • A FLWR expr may contain several fors and lets • each of these clauses may refere to variables bound in previous clauses • the result of the for/let sequence: • an ordered list of tuples of bound variables • number of tuples = product of the cardinalities of the node-lists returned by the expressions in the for clauses Notes 9: XQuery

  16. for/let clauses - Example 1 for$iin (1,2), $jin (3,4)return <tuple> <i>{$i}</i> <j>{$j}</j></tuple> Result: <tuple><i>1</i><j>3</j></tuple> <tuple><i>1</i><j>4</j></tuple> <tuple><i>2</i><j>3</j></tuple> <tuple><i>2</i><j>4</j></tuple> Notes 9: XQuery

  17. for/let clauses - Example 2 let $s := (<one/>, <two/>, <three/>) return <out>{$s}</out> Result: <out> <one/> <two/> <three/> </out> Notes 9: XQuery

  18. where clause • binding tuples generated by for and let clauses filtered by an optional where clause • those with a true condition are used to instantiate the return clause • the where clause may contain several predicates connected by and, or, and not() • usually refer to the bound variables • sequences treated as Booleans similarly to node-lists in XPath: empty ~ false; non-empty ~ true Notes 9: XQuery

  19. where clause • Variables bound by a for clause represent a single item -> scalar predicates, e.g. $p/color = ”Red” • Variables bound by a let clause may represent lists of nodes -> list-oriented predicates, e.g. avg($p/price) > 100 • a number of aggregation functions available: avg(), sum(), count(), max(), min() (also in XPath 1.0) Notes 9: XQuery

  20. return clause • The return clause generates the output of the FLWR expression • instantiated once for each binding tuple • often contains element constuctors, references to bound variables, and nested subexpressions Notes 9: XQuery

  21. Examples (from "XML Query Use Cases") • Assume: a document named ”bib.xml” containing of a list of books:<book>+ <title> <author>+ <publisher> <year> <price> Notes 9: XQuery

  22. List the titles of books published by Morgan Kaufmann after 1998 <recent-MK-books> { } </recent-MK-books> for $b indocument(”bib.xml”)//book where $b/publisher = ”Morgan Kaufmann” and $b/year gt 1998 return <book year="{$b/year}"> {$b/title} </book> Notes 9: XQuery

  23. Result could be... <recent-MK-books> <book year=”1999”> <title>TCP/IP Illustrated</title> </book> <book year=”2000”> <title>Advanced Programming in the Unix environment</title> </book> </recent-MK-books> Notes 9: XQuery

  24. List each publisher with the average price of its books for $p indistinct-value(document(”bib.xml”)//publisher) let $a := avg(document(”bib.xml”)/book[ publisher = $p]/price) return <publisher> { <name> {$p/text()}</name> , <avgprice> {$a} </avgprice> } </publisher> Notes 9: XQuery

  25. Invert the book-list structure <author_list>{ {-- group books by authors --} for $a indistinct-values(document(”bib.xml”)//author ) return <author> <name> {$a/text()} </name> for $b indocument(”bib.xml”)//book[ author = $a] return $b/title </author> } </author_list> Notes 9: XQuery

  26. Make an alphabetic list of publishers; for each publisher, list books (title & price) in descending order of price for $p indistinct-values(document(”bib.xml”)//publisher ) return <publisher> <name> {$p/text()} </name> {for $b indocument(”bib.xml”)/book[ publisher = $p] return <book> {$b/title , $b/price} </book> sortby(price descending)} </publisher> sortby(name) Notes 9: XQuery

  27. Queries on Document Order • Operators precedes and follows to express conditions on document order • The next example involves a surgical report with elements procedure, incisionand anesthesia • return a "critical sequence" that contains all contents between the 1st and 2nd incisions of the 1st procedure Notes 9: XQuery

  28. Computing a "critical sequence" <critical_sequence> { let $p := //procedure[1] for $n in $p/node() where $n follows ($p//incision)[1]and $n precedes ($p//incision)[2] return $n } </critical_sequence> Notes 9: XQuery

  29. Filtering • Function filter(Expr) • in XQuery core function library • returns shallow copies of the nodes selected by Expr • mutual order and hierarchy btw nodes the same as in input sequence Notes 9: XQuery

  30. Hierarchical Filtering • Only selected nodes remain; other levels are collapsed Notes 9: XQuery

  31. Table of contents for”cookbook.xml”, consisting of section titles (at any level of nesting) let $b := document(”cookbook.xml”) return <toc> { filter($b//(section | section/title | section/title/text()) ) } </toc> Notes 9: XQuery

  32. Querying relational data • Lots of data is stored in relational databases • should be able to access also this data • Example: suppliers and parts • Table S: supplier numbers (sno) and names (sname) • Table P: part numbers (pno) and descriptions (descrip) • Table SP: relationships between suppliers and the parts they supply, including the price (price) of each part from each supplier Notes 9: XQuery

  33. * * * A possible XML form for relations Notes 9: XQuery

  34. SQL vs. XQuery • SQL: • XQuery: SELECT pno FROM p WHERE descrip LIKE ’Gear%’ ORDER BY pno; for $p indocument(”p.xml”)//p_tuple wherestarts-with($p/descrip, ”Gear”) return $p/pno sortby(.) Notes 9: XQuery

  35. Grouping • Many relational queries involve forming data into groups and applying some aggregation function such as count or avg to each group • in SQL: GROUP BY and HAVING clauses • Example: Find the part number and average price for parts that have at least 3 suppliers Notes 9: XQuery

  36. Grouping: SQL SELECT pno, avg(price) AS avgprice FROM sp GROUP BY pno HAVING count(*) >= 3 ORDER BY pno; Notes 9: XQuery

  37. Grouping: XQuery for $pn in distinct-values(document(”sp.xml”)//pno) let $sp := document(”sp.xml”)//sp_tuple[ pno = $pn] where count($sp) >= 3 return <well_supplied_item> { $pn, <avgprice> {avg($sp/price)} </avgprice> } <well_supplied_item> sortby(pno) Notes 9: XQuery

  38. Joins • Example: Return a ”flat” list of supplier names and their part descriptions, in alphabetic order for $sp indocument(”sp.xml”)//sp_tuple, $p indocument(”p.xml”)//p_tuple[ pno = $sp/pno], $s in document(”s.xml”)//s_tuple[ sno = $sp/sno] return <sp_pair>{ $s/sname , $p/descrip }<sp_pair> sortby (sname, descrip) Notes 9: XQuery

  39. XQuery vs. XSLT • Could we express XQuery queries with XSLT? • In principle yes, always, (but could be tedious) • Ex: Partial XSLT simulation of FLWR expressions XQuery:for $x inExpr … can be expressed in XSLT as:<xsl:for-each select="Expr"> <xsl:variable name="x" select="."> … </xsl:for-each> Notes 9: XQuery

  40. XQuery vs. XSLT XQuery:let $y := Expr … corresponds directly to:<xsl:variable name="y" select="Expr" /> andwhereCondition …can be expressed as: <xsl:if test="Condition"> … </xsl:if> Notes 9: XQuery

  41. XQuery vs. XSLT • XQuery:returnElemConstructor • can be simulated with a corresponding XSLT template: • static fragments as such • enclosed expressions in element content, e.g. {$s/sname}become <xsl:copy-ofselect="$s/sname" /> Notes 9: XQuery

  42. XQuery vs. XSLT: Example XQuery: for $b indocument(”bib.xml”)//book where $b/publisher = ”MK” and $b/year > 1998 return <book year="{$b/year}"> {$b/title} </book> <xsl:template match="/"> <xsl:for-each select="document('bib.xml')//book"> <xsl:variable name="b" select="." /> <xsl:if test="$b/publisher='MK' and $b/year > 1998”> <book year="{$b/year}"> <xsl:copy-of select="$b/title" /> </book></xsl:if> </xsl:for-each></xsl:template>  Notes 9: XQuery

  43. XSLT for XQuery FLWR Expressions • NB: The sketched simulation is not complete: • Only two things can be done in XSLT for result tree fragments produced by templates: • insertion in result tree, and conversion to a string (with xsl:value-of) • Not possible to apply further operations to results (like, e.g., sorting in XQuery) Notes 9: XQuery

  44. Summary • XQuery • W3C working draft of an XML query language • vendor support?? • http://www.w3.org/XML/Querymentions about 20 prototype implementations or products that support (or will support) some version of XQuery • future?? • promising confluence of document and database research Notes 9: XQuery

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