XQuery 1.0

1
XQuery 1.0

• Ceng 352

2
Outline

• Introduction
• What is XQuery?
• Example queries
• Detail
• Expressions
• XPath
• FLOWR
• Joins
• Quantified Expressions
• Examples

3
What is XQuery?

• XQuery is the language for querying XML data
• XQuery for XML is like SQL for databases
• XQuery is built on XPath expressions
• XQuery is defined by the W3C
• XQuery is supported by all the major database
  engines (IBM, Oracle, Microsoft, etc.)
  
• XQuery will become a W3C standard - and
  developers can be sure that the code will work
  among different products
  

4
XQuery - Examples of Use

• Extract information to use in a Web Service
• Generate summary reports
• Transform XML data to XHTML
• Search Web documents for relevant information

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XQuery is Not (Yet) a Web Standard

• XQuery is compatible with several W3C standards,
  such as XML, Namespaces, XSLT, XPath, and XML
  Schema.
  
• However, XQuery 1.0 is not yet a W3C
  Recommendation (XQuery is a Working Draft).
  Hopefully it will be a recommendation in the near
  future.

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Books.xml
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How to Select Nodes From "books.xml"?

• Functions
• XQuery uses functions to extract data from XML
  documents.
  
• The doc() function is used to open the
  "books.xml" file
  
• doc("books.xml")
• Path Expressions
• XQuery uses path expressions to navigate through
  elements in an XML document.
  
• The following path expression is used to select
  all the title elements in the "books.xml" file
  
• doc("books.xml")/bookstore/book/titl
  e

Everyday Italian
Harry Potter
XQuery Kick Start
Learning XML
8
How to Select Nodes From "books.xml"?

• Predicates
• XQuery uses predicates to limit the extracted
  data from XML documents.
  
• The following predicate is used to select all the
  book elements under the bookstore element that
  have a price element with a value that is less
  than 30
• doc("books.xml")/bookstore/bookprice

Har
ry Potter J K. Rowling
2005 29.99
book
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How to Select Nodes From "books.xml" With FLWOR

• doc("books.xml")/bookstore/bookprice30/title
• all the title elements under the book elements
  that is under
  
• the bookstore element that have a price element
  with a value that is higher than 30.
  
• for x in doc("books.xml")/bookstore/book
• where x/price30
• return x/title
• for x in doc("books.xml")/bookstore/book where
  x/price30
  
• order by x/title
• return x/title

XQuery Kick Start
Learning XML
Learning XML
XQuery Kick Start
10
XQuery Conditional Expressions

• for x in doc("books.xml")/bookstore/book
• return
• if (x/_at_category"CHILDREN")
• then x/title/text()
• else x/title/text()
• Everyday Italian Harry
  Potter Learning XML
  XQuery Kick Start

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Detail
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XQuery Expressions

• Literals Hello 47 4.7
• Variables x
• Constructed sequences
• a, b means concatenation
• 5 to 8 means 5,6,7,8
• (1, (2, 3), 4) is the same as 1,2,3,4

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Path Expressions

• Result of path expression set of values that
  along with their containing elements/attributes
  match the specified path
  
• /bank/customer/customer-name
• Joe
• Ali
• /bank/customer/customer-name/text()
• Joe
• Ali

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Path expressions

• Initial / denotes the root of the document
• Path expressions are evaluated from left to
  right
  
• Selection predicates may follow any step in a
  path, in
  
• /bank/accountbalance400
• Attributes are accessed using _at_
• /bank/accountbalance400/_at_account-number

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XPath

• bib matches a bib element
• matches any element
• / matches the root element
• /bib matches a bib element under root
• bib/paper matches a paper in bib
• bib/paper1 matches first paper in bib
• bib/paperlast() matches last paper in bib
• bib//paper matches a paper in bib, at any
  depth
  
• //paper matches a paper at any depth
• paperbook matches a paper or a book
• _at_price matches a price attribute
• bib/book/_at_price matches price attribute in book,
  in bib
  

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Construction mode, Computation mode

• Element construction ...
• eg, John Smithx1234hone
  
• may include attribute bindings in the start tag
• eg, ...
• the content between the start and end tags (as
  well as the attribute values) is in construction
  mode
  
• to switch to computation mode, must use
• eg, 23 41 is
  equivalent to
  y3541
  

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FLWOR semantics

• FOR clause, LET clause generate list of tuples of
  bound variables
  
• By iterating over a set of nodes
• By binding a variable to the result of an
  expression
  
• WHERE clause applies a predicate to filter the
  tuples produced by FOR/LET
  
• ORDER BY clause imposes order on the surviving
  tuples
  
• RETURN clause is executed for each surviving tuple

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FOR v.s. LET

• FOR
• Binds node variables ? iteration
• LET
• Binds collection variables ? one value

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

...
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Joins

• FOR a in /bank/account,
• c in /bank/customer,
• d in /bank/depositor
• WHERE a/account-number d/account-number
• and
• c/customer-name d/customer-name
• RETURN c a

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Joins

• List all suppliers. If a supplier offers medical
  items, list the descriptions of the items
  
• FOR s IN document(suppliers.xml)//supplier
• ORDER BY s/name
• RETURN
• s/name,
• FOR ci IN document(catalog.xml)//itemsupp_no
  s/number,
  
• mi IN document(medical_items.xml)//itemnumbe
  r ci/item_no
  
• RETURN mi/description

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Quantified Expressions

• Existential Quantification
• Give me all books where Sailing and
  Windsurfing appear at least once 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

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Quantified Expressions

• Universal Quantification
• Give me all books where Sailing appears in
  every paragraph
  
• FOR b IN //book
• WHERE EVERY p IN b//para SATISFIES
  contains(p, Sailing)
  
• RETURN b/title

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XQuery Functions

• Predefined Functions
• avg, sum, count, max, min
• empty(), text()
• User defined Functions

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Examples
FOR p IN distinct(docu
ment("bib.xml")//publisher) LET b doc
ument("bib.xml")/bookpublisher p
WHERE count(b) 100 RETURN p

count a (aggregate) function that returns the
number of elms
Publishers who published more than 100 books
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Examples
Find books whose price is larger than average
LET aavg(document("bib.xml")/bib/book/_at_price)
FOR b in document("bib.xml")/bib/book
WHERE b/_at_price a
RETURN b
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Examples

• for b in document(books.xml)//book
• where b/author/firstname John
• and b/author/lastname Smith
• return
• b/title,
• b/price
• May return
• XML29.99ook
  
• DOM and SAX40e
  

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Examples

• for b in document(books.xml)//book
• where b/author/firstname John
• and b/author/lastname Smith
• return
• b/title,
• b/price
• Will return
• b/title,b/price
• b/title,b/price

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Examples Equivalent Query

• for b in document(books.xml)//book
• author/firstname
  John
  
• and author/lastname
  Smith
  
• return
• b/title,
• b/price

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Example

• let d document(depts.xml)//departmentname
  cse
  
• for s in d//gradstudent
• where s/gpa 3.5
• return
• s/name,s/gpa

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Example xml
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Example Aggregates

• Find all books with more than 3 authors
• FOR x IN document("bib.xml")/bib/book
• WHERE count(x/author)3
• RETURN x
• count a function that counts
• avg computes the average
• sum computes the sum
• distinct-values eliminates duplicates

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Same Query

• FOR x IN document("bib.xml")/bib/bookcount(autho
  r)3
  
• RETURN x

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Example

• For each author, return all book titles he/she
  wrote
  

What about duplicate authors?
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Example

• Same eliminate duplicate authors

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Example sorting

• Sort the expensive books by first author name,
  book title
  
• LET b document(bib.xml)//bookprice 100
• ORDER BY b/author1, b/title
• RETURN b

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Questions 1

•       for i in document("data/R-item
  s.xml")//item_tuple    where      contains(i/de
  scription, "Bicycle")    return        
               i/itemno             
  i/description           

List the item number and description of all
bicycles
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Question 2

• for u in document("data\R-users.xml")//user_tuple
  return           u/name
                 for b in document("data\R-bids.
  xml")//bid_tuple        where u/userid
  b/userid        order by b/itemno
          return                   b/item
  no          b/bid_date          b/bid   
                 

List all users, and for each user, list its
bidding item, bidding price, and date