Xquery

Summary of XQuery FLWR expressions FOR and LET expressions Collections and sorting Resource W3C recommendation:

Symbols.Denotes the current node...Denotes the parent of the current node. / Denotes the root node, or a separator between steps in a path. //Denotes descendants of the current attributes of the current node. *Denotes "any" (node with unrestricted name). [ ] Brackets enclose a Boolean expression that serves as a predicate for a given step. [n ] When a predicate consists of an integer, it serves to select the element with the given ordinal number from a list of elements.

FLWR (“Flower”) Expressions FOR... LET... FOR... LET... WHERE... RETURN...

FOR v.s. LET FOR Binds node variables  iteration LET Binds collection variables  one value

WHERE - continued AND, OR, and NOT usually contain references to bound variables Variables bound in FOR clause usually contain scalar predicates $p/color = "Red" Variables bound in LET clause usually used in list predicates avg($p/price) > 100

Operators Allows expressions to be constructed using prefix and infix operators Standard arithmetic and logical operators “=“ “!=“ “ ” “+” “-” “*” Many built-in functions

Operators in Expressions XQuery allows expressions to be constructed using prefix and infix operators (BEFORE, AFTER XQuery contains usual logical and arithmetic operators Also operators like UNION, INTERSECT, and EXCEPT

Quantifiers Tests for existence of some elements that satisfy a condition Also used to test whether all elements in a collection satisfy a condition Key words satisfies and contains

XQuery Find all book titles published after 1995: FOR $x IN document("bib.xml") /bib/book WHERE $x/year > 1995 RETURN $x/title FOR $x IN document("bib.xml") /bib/book WHERE $x/year > 1995 RETURN $x/title Result: abc def ghi

XQuery For each author of a book by Morgan Kaufmann, list all books she published: FOR $a IN distinct( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t FOR $a IN distinct( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t distinct = a function that eliminates duplicates

XQuery Result: Jones abc def Smith ghi

XQuery FOR $x in expr -- binds $x to each element in the list expr LET $x = expr -- binds $x to the entire list expr Useful for common subexpressions and for aggregations

XQuery count = a (aggregate) function that returns the number of elms FOR $p IN distinct(document("bib.xml")//publisher) LET $b := document("bib.xml")/book[publisher = $p] WHERE count($b) > 100 RETURN $p FOR $p IN distinct(document("bib.xml")//publisher) LET $b := document("bib.xml")/book[publisher = $p] WHERE count($b) > 100 RETURN $p

XQuery Find books whose price is larger than average: LET $a=avg( document("bib.xml") FOR $b in document("bib.xml") /bib/book WHERE > $a RETURN $b LET $a=avg( document("bib.xml") FOR $b in document("bib.xml") /bib/book WHERE > $a RETURN $b

FOR v.s. LET FOR $x IN document("bib.xml") /bib/book RETURN $x FOR $x IN document("bib.xml") /bib/book RETURN $x Returns:... LET $x := document("bib.xml") /bib/book RETURN $x LET $x := document("bib.xml") /bib/book RETURN $x Returns:...

Collections in XQuery Ordered and unordered collections /bib/book/author = an ordered collection Distinct(/bib/book/author) = an unordered collection LET $a = /bib/book  $a is a collection $b/author  a collection (several authors...) RETURN $b/author Returns:...

Collections in XQuery What about collections in expressions ?  list of n prices * 0.7  list of n numbers *  list of n x m numbers ?? * +  * + * !!

Sorting in XQuery FOR $p IN distinct(document("bib.xml")//publisher) RETURN $p/text(), FOR $b IN document("bib.xml")//book[publisher = $p] RETURN $b/title, SORTBY(price DESCENDING) SORTBY(name) FOR $p IN distinct(document("bib.xml")//publisher) RETURN $p/text(), FOR $b IN document("bib.xml")//book[publisher = $p] RETURN $b/title, SORTBY(price DESCENDING) SORTBY(name)

If-Then-Else FOR $h IN //holding RETURN $h/title, IF = "Journal" THEN $h/editor ELSE $h/author SORTBY (title) FOR $h IN //holding RETURN $h/title, IF = "Journal" THEN $h/editor ELSE $h/author SORTBY (title)

Existential Quantifiers FOR $b IN //book WHERE SOME $p IN $b//para SATISFIES contains($p, "sailing") AND contains($p, "windsurfing") RETURN $b/title FOR $b IN //book WHERE SOME $p IN $b//para SATISFIES contains($p, "sailing") AND contains($p, "windsurfing") RETURN $b/title

Universal Quantifiers FOR $b IN //book WHERE EVERY $p IN $b//para SATISFIES contains($p, "sailing") RETURN $b/title FOR $b IN //book WHERE EVERY $p IN $b//para SATISFIES contains($p, "sailing") RETURN $b/title

Group-By in Xquery ?? FOR $b IN document(" $y IN WHERE $b/publisher="Morgan Kaufmann" RETURN GROUPBY $y WHERE count($b) > 10 IN $y FOR $b IN document(" $y IN WHERE $b/publisher="Morgan Kaufmann" RETURN GROUPBY $y WHERE count($b) > 10 IN $y SELECT year FROM Bib WHERE Bib.publisher="Morgan Kaufmann" GROUPBY year HAVING count(*) > 10 SELECT year FROM Bib WHERE Bib.publisher="Morgan Kaufmann" GROUPBY year HAVING count(*) > 10  with GROUPBY Equivalent SQL 

Query Example: Return a flat list of supplier names and their part descriptions for the parts that are actually supplied, in alphabetic order.

JOINS in Relation 1ABC100 2DEF75 3GHI36 4JKL2 5MN O 0 P (part) pno descrip qnty 27IBMNY 35MSF T WSH 8LSNJAX 14AMDCA 51AJRBNA 24UFGNV S (supplier) sno name locat SP (Supplies) pno sno price

XML documents P.XML 1 ABC 100 S.XML 27 IBM NY SP.XML

JOINS in XQuery For $sp in document(“sp.xml”)//sp_tuple, $p in document(“p.xml”)//p_tuple[ pno = $sp/pno] $s in document(“s.xml”)//s_tuple [sno = $sp/sno] Return { $s/name, $p/descrip } sortby(sname, descrip)

Binding of Joins in Xquery $sp sp_tuple sp_tuple sp_tuple sp_tuple sp_tuple $p p_tuple p_tuple p_tuple p_tuple 1 ABC DEF 75 3 GHI 36 4 JKL 2 $s s_tuple s_tuple 27 IBM NY 35 MSFT WSH 14 AMD CA 24 UF GNV

Additional Issues Updates : not implemented Case sensitivity: all keywords are case sensitive Type checking: When being processed a query goes through two phases: Analysis Phase & Evaluation Phase. During the Analysis phase type checking occurs to provide early detection of type errors and to compute the type of the result. Querying XML documents with cycles

TCP/IP Illustrated Stevens W. Addison-Wesley Advanced Programming in the Unix environment Stevens W. Addison-Wesley The Economics of Technology and Content for Digital TV Gerbarg Darcy CITI Kluwer Academic Publishers

List books published by Addison-Wesley after 1991, including their year and title. { for $b in document(" where $b/publisher = "Addison-Wesley" and > 1991 return { $b/title } }

Expected Result TCP/IP Illustrated Advanced Programming in the Unix environment

Create a flat list of all the title-author pairs, with each pair enclosed in a "result" element. { for $b in document(" $t in $b/title, $a in $b/author return { $t } { $a } }

Expected Results TCP/IP Illustrated Stevens W.

For each book found at both bn.com and amazon.com, list the title of the book and its price from each source. { for $b in document(" $a in document(" where $b/title = $a/title return { $b/title } { $a/price/text() } { $b/price/text() } }

For each book that has at least one author, list the title and first two authors, and an empty "et- al" element if the book has additional authors. { for $b in document(" where count($b/author) > 0 return { $b/title } { for $a in $b/author[position()<=2] return $a } { if (count($b/author) > 2) then else () } }

Data on the Web Abiteboul Serge Buneman Peter

List the titles and years of all books published by Addison-Wesley after 1991, in alphabetic order. { for $b in document(" where $b/publisher = "Addison-Wesley" and > 1991 return { } { $b/title } sort by (title) }

Path Expression (Q) In the second chapter of the document named "zoo.xml", find the figure(s) with caption "Tree Frogs". document("zoo.xml")//chapter[2]//figure[caption = "Tree Frogs"] First step locates the root node of a document. Second step locates the second chapter of the document ( ordinal number) Third step finds figure elements occurring anywhere within the chapter, but retains those figure elements that have a caption with the value "Tree Frogs."

Element Constructors To generate a new element is to embed the element directly in a query using XML notation. (Q) Generate an element that has an "empid" attribute and nested and elements. John Smith Anthropologist

Element Constructors (Q) Generate an element that has an "empid" attribute. The value of the attribute and the content of the element are specified by variables that are bound in other parts of the query. {$name} {$job}

FLWR Expressions A FLWR (pronounced "flower") expression is constructed from FOR, LET, WHERE, and RETURN clauses, which must appear in a specific order. A FLWR expression binds values to one or more variables and then uses these variables to construct a result.

FLWR Expressions (Q) List each publisher and the average price of its books. FOR $p IN distinct(document("bib.xml")//publisher) LET $a := avg(document("bib.xml")//book[publisher = $p]/price) RETURN {$p/text()} {$a}

Sorting A sequence can be ordered by means of a SORTBY clause that contains one or more "ordering expressions." (Q)List all books with price greater than $100, in order by first author; within each group of books with the same first author, list the books in order by title. document("bib.xml")//book[price > 100] SORTBY (author[1], title)

Operators and Expressions ( Q) Prepare a "critical sequence" report consisting of all elements that occur between the first and second incision in the first procedure. { LET $p := //procedure[1] FOR $e IN //* AFTER ($p//incision)[1] BEFORE ($p//incision)[2] RETURN shallow($e) } The shallow function makes a shallow copy of a node, including attributes but not including subelements.

Conditional Expressions (Q) Make a list of holdings, ordered by title. For journals, include the editor, and for all other holdings, include the author. FOR $h IN //holding RETURN {$h/title, IF = "Journal") THEN $h/editor ELSE $h/author } SORTBY (title)

Quantified Expressions (Q) Find titles of books in which both sailing and windsurfing are mentioned in the same paragraph. FOR $b IN //book WHERE SOME $p IN $b//para SATISFIES (contains($p, "sailing") AND contains($p, "windsurfing")) RETURN $b/title

More Examples ibm.com/developerworks/xml/library/x -xquery.html