Query Languages

1
Query Languages

• Why a query language? Extracting, Restructuring,
  Integration, Browsing
• XML-QL
• http//www.w3.org/TR/NOTE-xml-ql
• http//db.cis.upenn.edu/XML-QL/
• XPATH (part of a query language)
• httpwww.w3.org/TR/xpath
• XSLT
• http//www.w3.org/TR/xslt
• http//www.mulberrytech.com/quickref/XSLTquickref
  .pdf

2
XQuery -- likely to gain acceptance...

• and, like other things in W3C, not necessarily
  the best. XQuery is a group of projects.
  General blather and informal specifications
• http//www.w3.org/XML/Query
• Ingredients
• A concrete syntax http//www.w3.org/TR/xquery
• Based on XPath http//www.w3.org/TR/xpath.html
• A formal semantics and algebra
  http//www.w3.org/TR/query-semantics/
• Some test cases http//www.w3.org/TR/xmlquery-use
  -cases

3
Query Languages and DTDs

• The DOM does not interact with DTDs (later
  levels may do this)
• For query languages there is almost no
  interaction (only to find out the names of IDs
  and IDREFs)
• XDUCE (developed at Penn) is the only
  well-developed language that uses DTDs as a type
  system. www.cis.upenn.edu/hahosoya/xduce/

4
XML-QL (XML Query Language)

• W3C proposal, August 1998
• authors
• Mary Fernandez ATT
• Dana Florescu INRIA
• Alon Levy Univ. of Washington
• Dan Suciu ATT
• Alin Deutsch Univ. of Pennsylvania

5
Address Book Revisited

• Caesar
• Caesar Imperator
• The Capitol
• Rome, OH 98765
• [phone]
• [phone]
• [phone]
• jc_at_forum.rome.org

6
XML-QL Pattern Matching
Find Caesars e-mail address where

Caesar
e
in http//db.cis.upenn.edu
/peter/address.xml construct e
jc_at_forum.rome.org
Data Extraction
7
XML-QL Constructing New XML Data
Whom can we contact electronically? where

g
e
in http//... construct
g
e

Caesar
Imperator jc_at_forum.rome.org

Brutus mb_at_philippi.com
...
Data Restructuring
8
XML-QL Joins
Who of our contacts was involved in a
movie? where
g
e
in
http//address.xml
t
g in
http//www.imdb.com construct
g
t
e
9
XML-QL Joins (contd)
Caesar
Imperator jc_at_forum.rome.orgere Asterix and Cleopatra

Dr. Strangelove
strangelov_at_love.the.bomb
Dr. Strangelove or How I Stopped
... ...
Data Integration
10
XML-QL Data Model

• Directed, labeled graph
• Tags represented as edge labels
• Sets of attribute name-value pairs as node labels
• Two models ordered and unordered

11
XML-QL Data Model (contd)

• Caesar
• Caesar Imperator
• The Capitol
• Rome, OH 98765
• [phone]
• [phone]
• [phone]
• jc_at_forum.rome.org

12
XSL (Extensible Stylesheet Language)

• W3C working draft by Adobe Systems
• Original purpose was to specify rendering of XML
  documents (mainly by Web browsers HTML)
• Consists of two parts
• an XML transformation language
• a formatting vocabulary denoting typographic
  abstractions (paragraph, page, rule, footer,
  etc.)

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The XSL Processor
original document
stylesheet
transformer
restructured document, elements are formatting
objects
plug in your favorite
formatter
presentation (other formats possible)
14
Formatting Example
original document
An example This is a
test. This is anothertest.p
transformed document
An
example This is a
test. This is
another

test.
15
Template Rules

• ly-templates/

16
Template Rules (contd)

where n
e con
struct n
e
17
XSL vs. XML-QL
XML-QL XSL
XML output no schema required data
extraction data restructuring data
integration schema browsing relational complete
18

• XPATH and XQuery

19
URLs -- XPath

• http//www.w3.org/TR/xpath
• This is the recommendation. Dense. Few
  examples. Difficult to extract the big picture
  from the morass of detail
• http//www.zvon.org/xxl/XPathTutorial/
• General/examples.html
• A tutorial with some simple examples. Maybe too
  simple. There are lots of tutorials on the web.

20
URLs -- XQuery

• http//www.w3.org/TR/xquery/
• The basic recommendation. Plenty of examples,
  so work through these first.
• http//www.w3.org/TR/query-semantics/
• A formal semantics for XQuery. Despite its
  forbidding title, it is remarkably readable. It
  also discusses a type system for XQuery.
• http//www.w3.org/TR/xmlquery-use-cases
• A bunch of example queries and their solution in
  XQuery (not surprising, since XQuery is
  Turing-complete!)

21
How to Identify nodes in a Tree -- Regular Path
Expressions
In the normal syntax of regular
expressions db.emps.emp db.(depts.dept.mgr
emps.emp) db._.name
Mary
Bill
John
N.B. Regular path expressions have nothing to do
with regular expresions in DTDs
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More examples
With the DTD MOTHER)
the regular path expression
(PERSON.MOTHER) identifies matrilineal
ancestry XPATH is a superset of a subset of
regular path expressions. (It cannot express
this set of nodes.) However, it is not limited
to moving down the tree.
23
XPath

• Primary goal to permit to access some nodes
  from a given document
• XPath main construct axis navigation
• An XPath path consists of one or more navigation
  steps, separated by /
• A navigation step is a triplet axis node-test
  list of predicates
• Examples
• /descendantnode()/childauthor
• /descendantnode()/childauthorparent/attribute
  booktitle XML2
• XPath also offers some shortcuts
• no axis means child
• // º /descendant-or-selfnode()/

24
XPath- child axis navigation

• author is shorthand for childauthor. Examples
• aaa -- all the child nodes labeled aaa (1,3)
• aaa/bbb -- all the bbb grandchildren of aaa
  children (4)
• /bbb all the bbb grandchildren of any child
  (4,6)
• . -- the context node / -- the root node

25
XPath- child axis navigation (cont)

• /doc -- all the doc children of the root
• ./aaa -- all the aaa children of the context node
  (equivalent to aaa)
• text() -- all the text children of the context
  node
• node() -- all the children of the context node
  (includes text and attribute nodes)
• .. -- parent of the context node
• .// -- the context node and all its descendants
• // -- the root node and all its descendants
• //para -- all the para nodes in the document
• //text() -- all the text nodes in the document
• _at_font the font attribute node of the context node

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Predicates

• 2 -- the second child node of the context node
• chapter5 -- the fifth chapter child of the
  context node
• last() -- the last child node of the context
  node
• chaptertitleintroduction -- the chapter
  children of the context node that have one or
  more title children whose string-value is
  introduction (the string-value is the
  concatenation of all the text on descendant text
  nodes)
• person.//firstname joe -- the person
  children of the context node that have in their
  descendants a firstname element with string-value
  Joe
• From the XPath specification
• NOTE If x is bound to a node set then x
  foo does not mean the same as not (x !
  foo) .

27
Unions of Path Expressions

• employee consultant -- the union of the
  employee and consultant nodes that are children
  of the context node
• For some reason person/(employeeconsultant) --as
  in regular path expressions -- is not allowed
• However person/node()boolean(employeeconsultant)
  is allowed!!
• From the XPATH specification
• The boolean function converts its argument to a
  boolean as follows
• a number is true if and only if it is neither
  positive or negative zero nor NaN
• a node-set is true if and only if it is non-empty
• a string is true if and only if its length is
  non-zero
• an object of a type other than the four basic
  types is converted to a boolean in a way that is
  dependent on that type

28
Axis navigation

• So far, nearly all our expressions have moved us
  down the by moving to child nodes. Exceptions
  were
• . -- stay where you are
• / go to the root
• // all descendants of the root
• .// all descendants of the context node
• All other expressions have been abbreviations for
  child e.g. childpara. childis an example of
  an axis
• XPath has several axes ancestor,
  ancestor-or-self, attribute, child, descendant,
  descendant-or-self, following, following-sibling,
  namespace, parent, preceding, preceding-sibling,
  self
• Some of these (self, parent) describe single
  nodes, others describe sequences of nodes.

29
XPath Navigation Axes(merci, Arnaud Sahuguet)
ancestor
following-sibling
preceding-sibling
self
child
attribute
following
preceding
namespace
descendant
30
XPath abbreviated syntax
(nothing) child _at_ attribute // /descendan
t-or-selfnode() . selfnode() .// descendan
t-or-selfnode .. parentnode() / (document
root)
31
XPath

• Reasonably widely adopted -- in XML-Schema and
  query languages.
• Neither more expressive nor less expressive than
  regular path expressions (cant do (ab) )
• Particularly messy in some areas
• defining order of results
• overloading of operations,
• e.g. chapter/title Introduction
• why not Introduction IN chapter/title ?

32
XQuery

• proposed by Chamberlin, Robbie and Florescu
• (from the authors slides)
• Leverage the most effective features of several
  existing and proposed query languages
• Design a small, clean, implementable language
• Cover the functionality required by all the XML
  Query use cases in a single language
• Write queries that fit on a slide

33
XQuery XPath comprehension syntax

• XML -QL
• Quilt

34
XQuery

35
Examples from XQuery
List the titles of books published by Morgan
Kaufmann in 1998. FOR b IN document("bib.xml")/
/book WHERE b/publisher "Morgan Kaufmann"
AND b/_at_year "1998" RETURN b/title
XPath expressions in orange
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DTD for Sample Document

• publisher, price )

37
Examples from XQuery (cont)
List each publisher and the average price of its
books. FOR p IN distinct(document("bib.xml")//p
ublisher) LET a avg( document("bib.xml")/
/bookpublisher p/price) RETURN
p/text()
a
LET binds a variable to a value. It does not
cause an iteration. Does this create a
(well-formed) XML document?
38
Examples from XQuery (cont)
List the publishers who have published more than
100 books. FOR p IN
distinct(document("bib.xml")//publisher) LET b
document("bib.xml")//bookpublisher
p WHERE count(b) 100 RETURN p

What about efficiency?
39
Examples from XQuery (cont)
Invert the structure of the input document so
that each distinct author element contains a
sequence of book-titles. FOR
a IN distinct(document("bib.xml")//author)
RETURN a/text()
FOR b IN document("bib.xml")//bookauthor
a RETURN b/title _list