Introduction to XQuery

1. Introduction to XQuery Bun Yue Professor, CS/CIS UHCL

2. W3C Recommendations • http://www.w3.org/TR/xquery/: W3C XQuery http://www.w3.org/TR/xmlquery-use-cases: XQuery use cases. • http://www.w3.org/TR/xquery-operators/: XQuery and XPath functions. • http://www.w3.org/TR/xpath-datamodel/: XQuery 1.0 and XPath 2.0 Data Model. • http://www.w3.org/TR/xpath20/: XPath 2.0. • http://www.w3.org/TR/xmlschema-1/: XML Schema Part 1: Structures. • http://www.w3.org/TR/xmlschema-2/: XML Schema Part 2: datatypes.

3. Introduction • XQuery is designed for effectively query and retrieve information from a diversified XML sources. • The XML sources can be one or more XML documents. • XQuery is derived from Quilt, and has borrowed features from XPath, XQL, SQL, etc.

4. Introduction • It is a functional language where a query is an expression. • There are three faces of the XQuery languages: • A "surface" syntax that programmers may probably use. • An XML-based syntax that machine may probably use (XQueryX). • A formal semantic that XQuery engine implementators use.

5. Introduction. • XQuery 1.0 extends XPath 2.0. • The type system of XQuery is based on XML Schema. • A limitation of XQuery: • No update or insert. • The basic building block of XQuery is expressions. (In this sense, like SQL, XQuery is not a full programming language.)

6. Comparing to SQL

7. Review of XPath 2.0 • The value of an expression is a sequence, which is an ordered list of items. • An item can be a node or of atomic value. • There are 7 node types: • Document • Element • Attribute • Comment • Text • Processing Instruction • Namespace

8. XQueryX For doc("census.xml")//person[@job="Athlete"] the corresponding XQueryX can be: <?xml version="1.0"?> <q:query xmlns:q="http://www.w3.org/2001/06/xqueryx">  <q:step q:axis="descendant-or-self">    <q:function q:name="document">      <q:constant q:datatype="xs:string">census.xml</q:constant> </q:function>    <q:predicatedExpr>      <q:identifier>person</q:identifier>      <q:predicate> <q:function q:name="equals"> <q:step q:axis="attribute"> <q:identifier>job</q:identifier>               </q:step>               <q:constant q:datatype="xs:string">Athlete</q:constant>          </q:function>      </q:predicate>    </q:predicatedExpr>  </q:step> </q:query>

9. Data Types • XQuery is strongly typed. • XQuery types are based on • XML Schema: using the namespace prefix xs and url: http://www.w3.org/2001/XMLSchema. • XPath functions and operators: using the namespace prefix xdt and url: http://www.w3.org/2004/07/xpath-datatypes

10. Types

11. Types • xdt:untyped is used to denote element nodes not yet validated. • xdt:untypedAtomic is used to denote atomic types that has not been assigned a more specific type.

12. Query • A query in XQuery is an expression for • reading XML documents or fragments and • returning a sequence of well-formed XML fragments • Everything in XQuery is an expression that is evaluated to a value.

13. Query expressions • Some common forms of XQuery expressions are (these appear in most tutorials): • path expressions • element constructors • FLWR or FLOWR (pronounced as "flower") expressions • list expressions • conditional expressions • quantified expressions • datatype expressions

14. More Queries • Examples of other expressions include: • primary expressions • sequence expressions • arithmetic expressions • logical expressions • comparison expressions • sorting expressions • validate expressions

15. Comments • XQuery comments are embedded within (: and :).

16. Functions • Supports a collection of about 200 built-in operators and functions to be used within expressions. • Input functions in XQuery include doc() and collection(). They are used to identify the sources of the XML documents.

17. Input Functions • Input functions: • doc() • collection().

18. Prolog • XQuery may have prologs for declarations. Examples: • Variable declarations • Function declarations • Base-URI declarations • Version declarations • Module import • …

19. Variable Declarations • Format: declare variable $name = expression; • E.g. declare variable $a := doc("census.xml")//person ;

20. Path Expressions • XQuery 1.0 is a superset of XPath 2.0. • An XPath expression is also an XQuery expression

21. Editix • Use “View > Windows > XQuery Builder” • For XQ files, use “XSLT/XQuery > Transform using an XQuery Request…” • Specify source xq file, xml file and output file. • Use .xml extension. If you use .txt extension, only text node contents are output.

22. Examples declare base-uri "whatever-path"; doc("bib.xml")/* Return basically bib.xml.

23. Example doc("bib.xml")//* Return many nodes (in a sequence). • Results are not well-formed.

24. Examples doc("bib.xml")//book[@year] count(doc("census.xml")//person)

25. Element Constructors • Element constructors can be used to construct XML elements. • If the name, attributes, and content of the element are all constants, the element constructor is based on standard XML notation and is called a direct element constructor (W3C).

26. Example The XQuery <authors> <author>Bun Yue</author> </authors> returns <authors> <author>Bun Yue</author> </authors>

27. Element Constructors • XQuery expressions can be embedded in the direct element constructors within a pair of curly braces, {}. • For the characters '{' and '}', use '{{' and '}}' respectively. • XQuery expressions may be separated by commas.

28. Example <authors><author>Bun Yue</author>{ doc("bib.xml")//author }</authors> Adds Bun Yue to the authors of bib.xml.

29. Computed Constructors • Computed constructors can also be used to declare nodes: • Use the keywords element, attribute, document, text, processing-instruction, comment, or namespace to declare the type of the nodes. • Specify the node names for those node types with names (element, attribute, processing instruction, and namespace nodes) • Use a pair of braces to define the content expressions. • Note the use of commas to separate expressions in the context.

30. Example (from W3C) element book { attribute isbn {"isbn-0060229357" }, element title { "Harold and the Purple Crayon"}, element author { element first { "Crockett" }, element last {"Johnson" } } }

31. Example (result) <book isbn="isbn-0060229357">    <title>Harold and the Purple Crayon</title>    <author>      <first>Crockett</first>      <last>Johnson</last>    </author> </book>

32. Dynamic Element Names • Computed expressions can be used to create elements with dynamic names.

33. Example <result> { for $author in doc("bib.xml")//author return element {$author/last/text()} { $author/first } } </result>

34. Example Result <?xml version="1.0" encoding="UTF-8"?> <result> <Stevens> <first>W.</first> </Stevens> <Stevens> <first>W.</first> </Stevens> <Abiteboul> <first>Serge</first> </Abiteboul> <Buneman> <first>Peter</first> </Buneman> <Suciu> <first>Dan</first> </Suciu> </result>

35. Example • Note that <first> is a child element. See the difference of: <result> { for $author in doc("bib.xml")//author return element {$author/last/text()} { $author/first/text() } } </result>

36. Example • This example may also result in a runtime error (as the value of <last> may not be suitable for a QName.

37. FLWOR expressions • FLWOR expressions are one of the most important constructs in XQuery. • You may compare with the SELECT statement of SQL.

38. FLWOR (W3C) [42]    FLWORExpr    ::=    (ForClause | LetClause)+ WhereClause? OrderByClause? "return" ExprSingle [43]    ForClause    ::=    "for" "$" VarName TypeDeclaration? PositionalVar? "in" ExprSingle ("," "$" VarName TypeDeclaration? PositionalVar? "in" ExprSingle)* [45]    LetClause    ::=    "let" "$" VarName TypeDeclaration? ":=" ExprSingle ("," "$" VarName TypeDeclaration? ":=" ExprSingle)* [123]    TypeDeclaration    ::=    "as" SequenceType [44]    PositionalVar    ::=    "at" "$" VarName [46]    WhereClause    ::=    "where" Expr [47]    OrderByClause    ::=    ("order" "by" | "stable" "order" "by") OrderSpecList [48]    OrderSpecList    ::=    OrderSpec ("," OrderSpec)* [49]    OrderSpec    ::=    ExprSingle OrderModifier [50]    OrderModifier    ::=    ("ascending" | "descending")? (("empty" "greatest") | ("empty" "least"))? ("collation" StringLiteral)?

39. FLWOR • FLWOR expressions allow: • For: Iteration through items in XPath 2.0 sequences. Create a tuple stream where each tuple contains a distinct binding for each variable to a distinct value. • Let: Variables binding • Where: Predicate application for inclusion in the iteration. • Order by: Ordering data set for the iteration. • Return: Constructing new result for returning.

40. For and Let • The for and let clauses produces a tuple stream. • A tuple consists of one or more bound variables. • A variable begins with the prefix $. • A bound variable is one that has been assigned a value.

41. Example declare base-uri “whatever”; let $a := doc("bib.xml")//author return <authors> { $a } </authors>

42. Example Results <?xml version="1.0" encoding="UTF-8"?> <authors> <author> <last>Stevens</last> <first>W.</first> </author> <author> <last>Stevens</last> <first>W.</first> </author> … </authors>

43. Example Note • In this example: • The tuple stream is composed of only one tuple. • The variable $b in this tuple is bound to the node sequence of 5 <author> nodes.

44. Example for $a in doc("bib.xml")//author return <authors> { $a } </authors>

45. Example Result <?xml version="1.0" encoding="UTF-8"?> <authors> <author> <last>Stevens</last> <first>W.</first> </author> </authors> <authors> <author> <last>Stevens</last> <first>W.</first> </author> </authors> … </authors>

46. Example Notes • In this example: • The tuple stream is composed of only five tuples. • The variable $b in this tuple is bound to one <author> node at a time.

47. Example for $a in doc("bib.xml")//author, $b in doc("bib.xml")//author return <count/>

48. Example Result <?xml version="1.0" encoding="UTF-8"?> <count/> <count/> <count/> <count/> … (: 25 counts :)

49. Example Note • The tuple stream is composed of only 25 tuples. • The 25 tuples are: • ($a: <author><last>Stevens</last><first>W.</first></author>, $b: <author><last>Stevens</last><first>W.</first></author>) • ($a: <author><last>Stevens</last><first>W.</first></author>, $b: <author><last>Stevens</last><first>W.</first></author>) • ($a: <author><last>Stevens</last><first>W.</first></author>, $b: <author><last>Abiteboul</last><first>Serge</first></author>) • …

50. Example for $a in doc("bib.xml")//author, $b in $a/last return <count />