1. XML Querying and Views Zachary G. Ives November 1, 2007
University of Pennsylvania
CIS 550 – Database & Information Systems
November 1, 2007
Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan

2. 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?

3. 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}

4. “Iterations” in XQuery
A series of (possibly nested) FOR statements assigning the results of XPaths to variables
for $root in document(“
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

5. Example XML Data root attribute p-i element Root ?xml dblp
text
?xml
dblp
mastersthesis
article
mdate
mdate
key
key
2002…
author
title
year
school
editor
title
journal
volume
year ee ee
2002…
1992
1997
ms/Brown92
The…
tr/dec/…
PRPL…
Digital…
db/labs/dec
Kurt P….
Univ….
Paul R.
SRC…

6. 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>{ }</key>
{ $i/crossref }
</smith-paper>

7. Another Example … … root attribute p-i element Root ?xml universities
text
?xml
universities
university …
mastersthesis
name
country
key …
author
title
year
school
Univ….
USA
1999
ms/Brown92
Univ….
PRPL…
Kurt P….

8. 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) … }

9. 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 } }

10. 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 for more details on the language

11. XQuery Works Well with Schema, and Validates Against It (Incl. Keys)
<xsd:schema xmlns:xsd="
<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 </xsd:key>
<xsd:keyref name=“schoolRef” refer=“schoolId">       <xsd:selector xpath=“./school”/>  <xsd:field xpath=“./text()"/> </xsd:keyref>
</xsd:element>
</xsd:schema>

12. 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

13. 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 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

14. 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)

15. 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:stylesheet>

16. Results of XSLT Stylesheet
<dblp>
<inproceedings>
<title>Paper1</title>
<author>Smith</author>
</inproceedings>
<author>Chakrabarti</author>
<author>Gray</author>
<title>Paper2</title>
</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>

17. 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 or the chapter we handed out for more details

18. 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

19. 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)
SQL:
CREATE VIEW V(A,B,C) AS
SELECT A,B,C FROM R WHERE R.A = “123”
XQuery: declare function V() as element(content)* {
for $r in doc(“R”)/root/tree,
$a in $r/a, $b in $r/b, $c in $r/c
where $a = “123”
return <content>{$a, $b, $c}</content> }
Using the views:
SELECT * FROM V, R WHERE V.B = 5 AND V.C = R.C
for $v in V()/content, $r in doc(“r”)/root/tree where $v/b = $r/b return $v

20. 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

21. 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”
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
SELECT * FROM V, R WHERE V.B = 5 AND V.C = R.C

22. 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)

23. 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:
We can ensure views are updatable by enforcing certain constraints (e.g., no aggregation), but this limits the kinds of views we can have R A B 1 2 S B C 2 4 3
R⋈S A B C 1 2 4 3
delete?

24. 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)

25. 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!