Transcript Initial Lecture

XML Querying and Views
Zachary G. Ives
University of Pennsylvania
CIS 550 – Database & Information Systems
November 1, 2007
Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan
Administrivia
Homework 4 due 11/7
 XQuery
Handout re: project to come shortly – before project plan will
be due
Review the Shanmugasundaram paper by Wednesday 11/7
 Post a 1-page summary to the newsgroup
 What problems did they address? Basic techniques? Strengths &
weaknesses of their methods?
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XQuery’s Basic Form
 Has an analogous form to SQL’s
SELECT..FROM..WHERE..GROUP BY..ORDER BY
 The model: bind nodes (or node sets) to variables; operate
over each legal combination of bindings; produce a set of
nodes
 “FLWOR” statement [note case sensitivity!]:
for {iterators that bind variables}
let {collections}
where {conditions}
order by {order-conditions} (older version was “SORTBY”)
return {output constructor}
3
“Iterations” in XQuery
A series of (possibly nested) FOR statements assigning the results of XPaths
to variables
for $root in document(“http://my.org/my.xml”)
for $sub in $root/rootElement,
$sub2 in $sub/subElement, …
 Something like a template that pattern-matches, produces a “binding
tuple”
 For each of these, we evaluate the WHERE and possibly output the
RETURN template
 document() or doc() function specifies an input file as a URI
 Old version was “document”; now “doc” but it depends on your XQuery
implementation
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Example XML Data
Root
?xml
2002…
p-i
element
dblp
article
mdate
key
author title year school
1992
ms/Brown92
key
editor title journal volume year ee ee
2002…
tr/dec/…
PRPL…
Kurt P….
attribute
text
mastersthesis
mdate
root
Digital…
Univ….
1997
The…
Paul R.
db/labs/dec
SRC…
http://www.
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Two XQuery Examples
<root-tag> {
for $p in document(“dblp.xml”)/dblp/proceedings,
$yr in $p/yr
where $yr = “1999”
return <proc> {$p} </proc>
} </root-tag>
for $i in document(“dblp.xml”)/dblp/inproceedings[author/text() = “John Smith”]
return <smith-paper>
<title>{ $i/title/text() }</title>
<key>{ $i/@key }</key>
{ $i/crossref }
</smith-paper>
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Another Example
Root
?xml
name
country
…
…
p-i
element
mastersthesis
key
author title year
Univ….
attribute
text
universities
university
root
school
USA
1999
ms/Brown92
PRPL…
Univ….
Kurt P….
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What If Order Doesn’t Matter?
By default:
 SQL is unordered
 XQuery is ordered everywhere!
 But unordered queries are much faster to answer
XQuery has a way of telling the query engine to avoid
preserving order:
 unordered {
for $x in (mypath) …
}
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Querying & Defining Metadata –
Can’t Do This in SQL
Can get a node’s name by querying node-name():
for $x in document(“dblp.xml”)/dblp/*
return node-name($x)
Can construct elements and attributes using computed names:
for $x in document(“dblp.xml”)/dblp/*,
$year in $x/year,
$title in $x/title/text(),
element node-name($x) {
attribute {“year-” + $year} { $title }
}
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XQuery Wrap-up
 XQuery is very SQL-like, but in some ways cleaner
and more orthogonal
 It is based on paths and binding tuples, with
collections and trees as its first-class objects
 See www.w3.org/TR/xquery/ for more details on the
language
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XQuery Works Well with Schema,
and Validates Against It (Incl. Keys)
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:complexType name=“ThesisType">
<xsd:attribute name=“key" type="xsd:string"/>
…
<xsd:sequence>
<xsd:element name=“author" type=“xsd:string"/>
...
<xsd:element name=“school" type=“xsd:string”/>
…
</xsd:sequence>
</xsd:complexType>
<xsd:element name=“mastersthesis" type=“ThesisType">
<xsd:key name=“mtId">
<xsd:selector xpath=“.”/> <xsd:field xpath="@key"/>
</xsd:key>
<xsd:keyref name=“schoolRef” refer=“schoolId">
<xsd:selector xpath=“./school”/> <xsd:field xpath=“./text()"/>
</xsd:keyref>
</xsd:element>
</xsd:schema>
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A Problem
 We frequently want to reference data in a way that differs
from the way it’s stored




XML data  HTML, text, etc.
Relational data  XML data
Relational data  Different relational representation
XML data  Different XML representation
 Generally, these can all be thought of as different views over
the data
 A view is a named query
 Let’s start with a special presentation language for XML  HTML
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XSL(T): XML  “Other Stuff”
 XSL (XML Stylesheet Language) is actually divided into two
parts:
 XSL:FO: formatting for XML
 XSLT: a special transformation language
 We’ll leave XSL:FO for you to read off www.w3.org, if you’re
interested
 XSLT is actually able to convert from XML  HTML, which
is how many people do their formatting today
 Products like Apache Cocoon generally translate XML  HTML on
the server side
 Your browser will do XML  HTML on the client side
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A Different Style of Language
 XSLT is based on a series of templates that match
different parts of an XML document
 There’s a policy for what rule or template is applied if
more than one matches (it’s not what you’d think!)
 XSLT templates can invoke other templates
 XSLT templates can be nonterminating (beware!)
 XSLT templates are based on XPath “match”es, and
we can also apply other templates (potentially to
“select”ed XPaths)
 Within each template, we describe what should be output
 (Matches to text default to outputting it)
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An XSLT Stylesheet
<xsl:stylesheet version=“1.1”>
<xsl:template match=“/dblp”>
<html><head>This is DBLP</head>
<body>
<xsl:apply-templates />
</body>
</html>
</xsl:template>
<xsl:template match=“inproceedings”>
<h2><xsl:apply-templates select=“title” /></h2>
<p><xsl:apply-templates select=“author”/></p>
</xsl:template>
…
</xsl:stylesheet>
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Results of XSLT Stylesheet
<dblp>
<inproceedings>
<title>Paper1</title>
<author>Smith</author>
</inproceedings>
<inproceedings>
<author>Chakrabarti</author>
<author>Gray</author>
<title>Paper2</title>
</inproceedings>
</dblp>
<html><head>This Is DBLP</head>
<body>
<h2>Paper1</h2>
<p>Smith</p>
<h2>Paper2</h2>
<p>Chakrabarti</p>
<p>Gray</p>
</body>
</html>
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What XSLT Can and Can’t Do
 XSLT is great at converting XML to other formats
 XML  diagrams in SVG; HTML; LaTeX
 …
 XSLT doesn’t do joins (well), it only works on one XML file at a time, and
it’s limited in certain respects
 It’s not a query language, really
 … But it’s a very good formatting language
 Most web browsers (post Netscape 4.7x) support XSLT and XSL
formatting objects
 But most real implementations use XSLT with something like Apache
Cocoon – compatible with more browsers
 You should use XSL/XSLT to format the forms and pages for your
projects – see www.w3.org/TR/xslt or the chapter we handed out for
more details
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Other Forms of Views
XSLT is a language primarily designed from going from
XML  non-XML
Obviously, we can do XML  XML in XQuery
… Or relations  relations
… What about relations  XML and XML  relations?
Let’s start with XML  XML, relations  relations
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Views in SQL and XQuery
 A view is a named query
 We use the name of the view to invoke the query
(treating it as if it were the relation it returns)
Using the views:
SQL:
CREATE VIEW V(A,B,C) AS
SELECT *
SELECT A,B,C FROM R
FROM V, R
WHERE R.A = “123”
WHERE V.B = 5
AND V.C = R.C
XQuery:
declare function V() as element(content)* {
for $v in V()/content,
for $r in doc(“R”)/root/tree,
$r in doc(“r”)/root/tree
$a in $r/a, $b in $r/b, $c in $r/c
where $v/b = $r/b
where $a = “123”
return $v
return <content>{$a, $b, $c}</content>
}
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What’s Useful about Views
Providing security/access control
 We can assign users permissions on different views
 Can select or project so we only reveal what we want!
Can be used as relations in other queries
 Allows the user to query things that make more sense
Describe transformations from one schema (the base relations)
to another (the output of the view)
 The basis of converting from XML to relations or vice versa
 This will be incredibly useful in data integration, discussed soon…
Allow us to define recursive queries
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Materialized vs. Virtual Views
 A virtual view is a named query that is actually re-computed
every time – it is merged with the referencing query
CREATE VIEW V(A,B,C) AS
SELECT A,B,C FROM R
WHERE R.A = “123”
SELECT *
FROM V, R
WHERE V.B = 5 AND V.C = R.C
 A materialized view is one that is computed once and its
results are stored as a table




Think of this as a cached answer
These are incredibly useful!
Techniques exist for using materialized views to answer other queries
Materialized views are the basis of relating tables in different schemas
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Views Should Stay Fresh
 Views (sometimes called intensional relations) behave,
from the perspective of a query language, exactly
like base relations (extensional relations)
 But there’s an association that should be maintained:
 If tuples change in the base relation, they should change in
the view (whether it’s materialized or not)
 If tuples change in the view, that should reflect in the base
relation(s)
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View Maintenance and
the View Update Problem
 There exist algorithms to incrementally recompute a
materialized view when the base relations change
 We can try to propagate view changes to the base relations
 However, there are lots of views that aren’t easily updatable:
R
B C
R⋈S A B C
1 2
2 4
1 2 4
2 2
2 3
1 2 3
A B
S
delete?
2 2 4
 We can ensure views are updatable
by enforcing certain constraints (e.g., no aggregation), 2 2 3
but this limits the kinds of views we can have
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Views as a Bridge between Data Models
A claim we’ve made several times:
“XML can’t represent anything that can’t be expressed in in
the relational model”
If this is true, then we must be able to represent XML
in relations
Store a relational view of XML
(or create an XML view of relations)
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A View as a Translation between
XML and Relations
 You’ll be reviewing the most-cited paper in this area
(Shanmugasundaram et al), and there are many more
(Fernandez et al., …)
 Techniques already making it into commercial
systems
 XPERANTO at IBM Research will appear in DB2 v9
 SQL Server has some XML support; Oracle is also doing
some XML
 … Next time you’ll see how it works!
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