Advanced Database Technologies XQuery

Transcription

1 Advanced Database Technologies XQuery Christian Grün Database & Information Systems Group

2 Introduction What is XQuery? query language (more than a) counterpart to SQL functional language general purpose language? information processing language Alternatives XPath: subset XSLT: comparable features, but specialized on transformations; more verbose SQL 2005: combines SQL and XQuery 2

3 Introduction Features browsing, selecting and filtering data sorting, grouping, aggregating contents working across many documents (collections) XQuery Update: modify collections XQuery Full Text: information retrieval in XML corpora brings databases and information retrieval closer together 3

4 Introduction Functional Language prominent examples: Scheme, Haskell, OCAML, R based on the evaluation of functions: one input, one output no side effects (that would change the state of a program) higher-order functions: functions as arguments iteration recursion XQuery 3.0 is a fully functional language (although some extensions break the rules) 4

5 XQuery Data Model Sequences all XQuery expressions evaluate to sequences lists of items with zero or more values: ("different",2,"types"); <H>U</H>; () sequences without values: empty sequences sequences are ordered: (1,2) (2,1) may contain duplicates (in contrast to XPath) cannot be nested: (1,(2,3),4) (1,2,3,4) 5

6 XQuery Data Model Data Types items: atomic values or nodes nodes: XML constructs elements, attributes,... atomic values: integer, double, string,... sequences: ordered lists of zero or more items Brundage [2004] 6

7 XQuery Data Model XML Node Types document nodes <element> elements <e id="0">text</e> <!-- important --> attributes <?php echo "pi";?> texts </element> comments processing instructions text test.xml element e <! > <?...?> id="0" 7

8 XQuery Data Model Atomic Values the complete picture... in practice, a few types suffice: xs:integer, xs:decimal, xs:double xs:string xs:boolean xs:date, xs:time xs:duration what is xs:untypedatomic? XQuery Data Model,

9 XQuery Data Model Untyped Data no schema information is available no explicit type has been assigned: data(<xml>text</xml>), data(doc("doc.xml")) Typed Data XQuery primitives, explicit assignment: xs:decimal: xs:string: "!" xs:date: xs:date(" ") 9

10 XQuery Data Model Creating Atomic Values literals (see later): "string", 0.98 explicit type assignment: xs:string("string"), xs:byte(127), xs:datetime(" t23:59:59") resulting from function call: true() xs:boolean max((1, 2, 5, 10, 20, 50, 100)) xs:integer 10

11 Expressions Primary Expressions literals, variables, function calls, parentheses Arithmetics, Comparisons numeric and date comparisons comparisons based on values or document order Logical Expressions Boolean operators Sequence Operators set-based operations Constructors direct and computed node construction 11

12 Primary Expressions Literals (Constants) string: "example" integer: 1234 decimal: double: 12.3e4 Variables dollar syntax with optional prefixes: $a, $pf:b Function Calls function name, arguments: doc("input.xml") local:function(99) Comments enclosed by (: smileys (: can be nested :) :) 12

13 Arithmetic Expressions Addition, Subtraction numbers: dates: date(" ") + xs:duration("p9y") strings: concat("and ","some ","more") Multiplication, (Integer) Division, Modulo 4 * 5 div 6, 7 idiv 8, 9 mod 10 13

14 Comparison Expressions Value Comparisons operators: eq, ne, lt, le, gt, ge always compares single atomic values () is returned if either operand is empty sequence operands need to have the same type Examples 3 gt 4 () ne () "a" le "b" "123" eq 123 <x>1</x> lt 2 (1,2,3) eq (1,2,3) 14

15 Comparison Expressions General Comparisons operators: =,!=, <, <=, >, >= compares all sequence values to each other returns true if at least one combination yields true untyped values are implicitly cast to relevant type Examples 3 > 4 () = () ()!= () "a" <= "b" "123" = 123 <x>1</x> < 2 (1,2,3) > (3,2,1) 15

16 Comparison Expressions Node Comparisons operators: is, <<, >> compares nodes based on their identity, which represents the node order document can be opened multiple times identity differs for constructors Examples doc("input.xml") is doc("input.xml") doc("input.xml")/a << doc("input.xml")/b doc("x") >> doc("x") <x/> is <x/> <x/> << <x/> <x/> >> <x/> 16

17 Conditional Expressions If/then/else Expression Syntax: if(test) then expr1 else expr2 nested syntax: if(test1) then expr1 else if(test2) then expr1 else expr2 Effective Boolean Value Test is evaluated as boolean: strings: string length > 0 numbers: different to 0 boolean: adopted as is empty sequence:false node sequences: true other items/sequences: 17

18 Conditional Expressions Switch Expression introduced with XQuery 3.0 Syntax: switch(test) case case1 return expr1... default return defaultexpr Typeswitch Expression tests type of input value Syntax: typewitch(test) case type return expr1... default return defaultexpr 18

19 Sequence Types Type Cast convert types to another type: "1" cast as xs:integer xs:integer("1") = 1 "0815" cast as xs:byte Type Cast Test check if cast is possible: "0815" castable as xs:byte Instance Check check types: (1,2,3) instance of xs:integer+ 19

20 Logical Expressions And/or Expressions Syntax: expr1 and expr2 expr1 or expr2 performs boolean tests expressions can be arbitrarily nested Functions true() / false() creates boolean value boolean(expr) converts expr to effective boolean value not(expr) inverses boolean empty(expr) / exists(expr) checks if sequence contains items 20

21 Sequence Expressions Quantifiers Syntax: some var in expr satisfies test every var in expr satisfies test Example some $v in (0, 2, 4, 5) satisfies $v mod 2 = 1 Combining Sequences Syntax: seq1 union seq2 seq1 intersect seq2 seq1 except seq2 Range Expression Syntax: min to max reverse(1 to 100) 21

22 Filters and Predicates Predicates [...] used to filter expressions evaluated as effective boolean values applied from left to right Context Item. references currently processed item (one by one) Examples "A"["will be true"] //X[text = "relevant"] (1 to 10)[. mod 2 = 1] (1 to 10)[.*. > 20] [.+. < 20] (1 to 20)[.*. > 20 and.+. < 20] 22

23 Filters and Predicates Positional Predicates filters items based on current context position comparable to array offsets predicate order is important! Functions position() yields current pos. last() chooses last position Examples (1 to 10)[1] (1 to 10)[2][1] (1 to 10)[1][2]...[position() = 1]...[position() = 1 to 5]...[last()] 23

24 FLWOR Expressions clauses: For, Let, Where, Order by, Return step by step evaluation of sequences iterative binding of variables Examples for $n in reverse(1 to 10) for $n in 1 to 10 let $p := $n * $n where $n * $n > 40 where $p > 40 order by $n descending return $p return $n * $n What is the output of the two expressions? 24

25 FLWOR Expressions Nested FLWOR Expressions for $a in 1 to 4 for $b in 1 to 3 where $a * $b > 8 return for $c in 1 to 2 let $d := ($a, $b, $c) return <hit>{ $d }</hit> TIMTOWTDI (Larry Wall) Equivalent Example for $c in 1 to 2 for $b in 1 to 3 for $a in 1 to 4 where $a * $b > 8 order by $a, $b, $c return <hit>{ ($a, $b, $c) }</hit> 25

26 FLWOR Expressions XQuery vs XPath XQuery for $movie in doc("library.xml")//movie return $movie XPath doc("library.xml")//movie XQuery XPath for $movie in doc("library.xml")//movie let $title := $movie/title where $title = "Juha" return $movie doc("library.xml")//movie[title = "Juha"] 26

27 FLWOR Expressions XQuery vs XPath XQuery for $movie in doc("library.xml")//movie let $title := $movie/title where starts-with($title, "La ") order by $title return <hit id="{ }">{ $title }</hit> XPath... doc("library.xml")//movie[starts-with(title, "La ")] No XPath equivalent for order/group by, node construction,... 27

28 Variable Assignments Global Variables defined in the query header: declare variable $var := expr; may also be defined as external Local Variables part of FLWOR, quantifier, typeswitch and someother expressions Shadowing variables are always final variables with the same name will be shadowed Example declare variable $a:= 1; let $a:= 12 return ( (let $a := $a * $a return $a), $a), $a 28

29 Try/Catch Expression Handling Errors supported since XQuer y 3.0 provides error handling Generic Case try { expr1 } catch * { expr2 } Catching Specific Errors try { expr1 } catch code { expr2 } Catching Additional Information try { expr1 } catch code($code, $desc) { expr2 } 29

30 Constructors Direct Constructors XML nodes can be constructed on-the-fly iterative binding of variables simple XML snippets (without prolog and CDATA blocks) are valid XQuery expressions Example creation of an element, attribute, and text node: <node1 id="0">direct Constructor</node1> 30

31 Constructors Embedding XQuery Expressions arbitrary XQuery expressions can be embedded in curly brackets Example <result>{ for $n in 1 to 10 return <counter id="{ $n }">{ format-integer($n, "w") }</counter> }</result> 31

32 Constructors Computed Constructors more verbose syntax used for constructing dynamic XML contents Example creation of an element, attribute and text node: element { concat("node", 1) } { attribute { "id" } { "0" }, text { "Computed Constructor" } } 32

33 Functions Function Libraries XQuery core is extended by function libraries by default, about 150 functions are available default functions use (optional) fn prefix: fn:true() true() BaseX offers appr. 70 additional functions, which use custom prefixes: Java binding can be used to access arbitrary Java functions 33

34 Input Functions Open document node doc("doc.xml") Open collection (sequence of document nodes) collection("docs") Open docs from database db:open("database") Convert XML string to node parse-xml("<xml/>") Parse text file file:read-text("file.txt") Parse binary file file:read-binary( "file.bin") Open zipped XML document zip:xml-entry( "archive.zip", "doc.xml") 34

35 Output Functions Serialize XML to string serialize(//product) Write to file file:write( "doc.xml", <xml/>) file:write( "range.txt", 1 to 100) file:write-binary( "file.bin", xs:base64binary("selq")) Measure performance util:ms(1 to ) util:mb(1 to 1000,true()) Show debugging output for $i in 1 to 10 return trace($i,"debug:") Raise error if ($n > 1000) then error() else $n 35

36 String Functions Find substrings contains("chekhov", "hek") Return string length string-length("camus") Change case lower-case("dostoyevsky") upper-case("bukowski") Concatenate strings concat("nie","tz","sche") Regex match matches("kafka","kaf.*") Replace with regex replace("axb","[a-z]","") Tokenize via regex tokenize("a b c","\s") Join strings string-join( ("a","b","c")," ") 36

37 Sequence Functions Remove duplicates distinct-values(//year) Find index positions index-of( ("o","x","o"), "X") Remove items remove(("o","x","o"), "X") Return subsequence subsequence((4,5,6), 2, 1) Reverse sequence reverse(1 to 100) Return first item (used in recursive functions) head(1 to 100) Second and following items tail(1 to 100) In XQuery, numbering always starts from 1! 37

38 Numeric Functions Count sequence length count(//title) Minimum/maximum values min(//title/year) Calculate sum sum(1 to 2345) Calculate average avg(for $p in //person return Absolute number abs(-1) Rounded number round(2.345, 2) Square root math:sqrt(100) Random number xs:int(math:random() * 5) Number π math:pi() 38

39 Functions User-defined Functions existing functions can be extended by declaring your own functions makes the code better readable and reusable by default, the local prefix is reserved used for user-defined functions Example declare function local:double($d as xs:double) { $d * 2 }; local:double(2.0) 39

40 40 References XQuery: Search Across a Variety of XML Data Priscilla Walmsley, 2007 XQuery: The XML Query Language Michael Brundage, 2004 XQuery From the Experts Howard Katz (ed.), 2004 XQuery Data Model. W3C, XQuery 3.0. W3C, XQuery Functions