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XQuery: Advanced Database Systems Lecture Notes, Slides of Database Management Systems (DBMS)

Duke UniversityDatabase Management Systems (DBMS)

These lecture notes cover various aspects of xquery, an expression language for querying and transforming xml documents. Topics include xpath, flwr expressions, subqueries, existentially and universally quantified expressions, aggregation, sorting, and more. Students will learn how to write xquery expressions to retrieve and manipulate data from xml documents.

Typology: Slides

2011/2012

Uploaded on 01/28/2012

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arold 🇺🇸

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1
XQuery
CPS 216
Advanced Database Systems
2
Announcements (March 21)
Midterm has been graded
Homework #3 will be assigned next Tuesday
Reading assignment due next Wednesday
XML processing in Lore (VLDB 1999) and Niagara
(VLDB 2003)
Project milestone 2 due next Thursday
3
XQuery
XPath + full-fledged SQL-like query language
XQuery expressions can be
XPath expressions
FLWR (C) expressions
Quantified expressions
Aggregation, sorting, and more…
An XQuery expression in general can return a new
result XML document
Compare with an XPath expression, which always
returns a sequence of nodes from the input document or
atomic values (boolean, number, string, etc.)
pf3
pf4
pf5

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XQuery

CPS 216

Advanced Database Systems

2

Announcements (March 21)

™ Midterm has been graded

™ Homework #3 will be assigned next Tuesday

™ Reading assignment due next Wednesday

ƒ XML processing in Lore ( VLDB 1999) and Niagara

( VLDB 2003)

™ Project milestone 2 due next Thursday

3

XQuery

™ XPath + full-fledged SQL-like query language

™ XQuery expressions can be

ƒ XPath expressions

ƒ FLWR (C) expressions

ƒ Quantified expressions

ƒ Aggregation, sorting, and more…

™ An XQuery expression in general can return a new

result XML document

ƒ Compare with an XPath expression, which always

returns a sequence of nodes from the input document or

atomic values (boolean, number, string, etc.)

A simple XQuery based on XPath

Find all books with price lower than $

doc(“bib.xml”)/bibliography/book[@price<50]

™ Things outside {}’s are copied to output verbatim

™ Things inside {}’s are evaluated and replaced by the results

ƒ doc(“bib.xml”) specifies the document to query

ƒ The XPath expression returns a sequence of book elements

ƒ These elements (including all their descendents) are copied to

output

5

FLWR expressions

™ Retrieve the titles of books published before 2000,

together with their publisher

{ for $b in doc(“bib.xml”)/bibliography/book let $p := $b/publisher where $b/year < 2000 return { $b/title } { $p } }

™ for: loop ƒ $b ranges over the result sequence, getting one item at a time ™ let: assignment ƒ $p gets the entire result of $b/publisher (possibly many nodes) ™ where: filter condition ™ return: result structuring ƒ Invoked in the “innermost loop,” i.e., once for each successful binding of all query variables

6

An equivalent formulation

™ Retrieve the titles of books published before 2000,

together with their publisher

{ for $b in doc(“bib.xml”)/bibliography/book[year<2000] return { $b/title } { $b/publisher } }

An explicit join

™ Find pairs of books that have common author(s)

{ for $b1 in doc("bib.xml")//book for $b2 in doc("bib.xml")//book where $b1/author = $b2/author return {$b1/title} {$b2/title} }

11

Existentially quantified expressions

(some $ var in collection satisfies condition )

ƒ Can be used in where as a condition

™ Find titles of books in which XML is mentioned in

some section

{ for $b in doc(“bib.xml”)//book where (some $section in $b//section satisfies contains(string($section), “XML”)) return $b/title }

12

Universally quantified expressions

(every $ var in collection satisfies condition )

ƒ Can be used in where as a condition

™ Find titles of books in which XML is mentioned in

every section

{ for $b in doc(“bib.xml”)//book where (every $section in $b//section satisfies contains(string($section), “XML”)) return $b/title }

Aggregation

™ List each publisher and the average prices of all its books

{ for $pub in distinct-values(doc(“bib.xml”)//publisher) let $price := avg(doc(“bib.xml”)//book[publisher=$pub]/@price) return {$pub} {$price} }

ƒ distinct-values( collection ) removes duplicates by value

  • If the collection consists of elements (with no explicitly declared types), they are first converted to strings representing their text contents

ƒ avg( collection ) computes the average of collection (assuming each

item in collection can be converted to a numeric value)

14

Sorting (a brief history)

™ XPath always returns a sequence of nodes in original

document order

™ for loop will respect the ordering in the sequence

™ August 2002

ƒ Introduce an operator sort by ( sort-by-expression-list ) to output

results in a user-specified order

ƒ Example: list all books with price higher than $100, in order by

first author; for books with the same first author, order by title

{

doc(“bib.xml”)//book[@price>100]

sort by (author[1], title)

}

15

Tricky semantics

™ List titles of all books, sorted by their prices

{ (doc(“bib.xml”)//book sort by (@price))/title }

ƒ What is wrong?