Querying XML – ConcludedIntroduction to Views

Zachary G. IvesUniversity of Pennsylvania

CIS 550 – Database & Information Systems

October 9, 2003

Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan

2

Administrivia

Final exam will be take-home, given out the last day of class and due Dec. 18th at 1PM

We’ll try to post HW3 answers on the Web shortly

Project page on web site now links to more detailed descriptions and references for each project

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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:for {iterators that bind variables}let {collections}where {conditions}order by {order-conditions}return {output constructor}

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“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

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Two XQuery Examples

<root-tag> {for $p in document(“dblp.xml”)/dblp/proceedings, $yr in $p/yrwhere $yr = “1999”return <proc> {$p} </proc>

} </root-tag>

for $i in doc (“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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Nesting in XQuery

Nesting XML trees is perhaps the most common operationIn XQuery, it’s easy – put a subquery in the return clause

where you want things to repeat!

for $u in doc(“dblp.xml”)/universities

where $u/country = “USA”

return <ms-theses-99>

{ $u/title} {

for $mt in $u/../mastersthesis

where $mt/year/text() = “1999”

return $mt/title }

</ms-theses-99>

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Collections & Aggregation in XQuery

In XQuery, many operations return collections XPaths, sub-XQueries, functions over these, … The let clause assigns the results to a variable

Aggregation simply applies a function over a collection, where the function returns a value (very elegant!)

let $allpapers := doc(“dblp.xml”)/dblp/articlereturn <article-authors>

<count> {fn:count(fn:distinct-values($allpapers/authors)) } </count>

{ for $paper in doc(“dblp.xml”)/dblp/articlelet $pauth := $paper/authorreturn <paper> {$paper/title}

<count> { fn:count($pauth) } </count> </paper>

} </article-authors>

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Sorting in XQuery

SQL actually allows you to sort its output, with a special ORDER BY clause (which we haven’t discussed)

XQuery borrows this idea In XQuery, what we order is the sequence of

“result tuples” output by the return clause:

for $x in doc(“dblp.xml”)/proceedingsorder by $x/title/text()return $x

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Distinct-ness

XQuery has a notion that DISTINCT-ness happens as a function over a collection But since we have nodes, we can do duplicate

removal according to value or node Can do fn:distinct-values(collection) to remove

duplicate values, or fn:distinct-nodes(collection) to remove duplicate nodes

for $years in fn:distinct-values(doc(“dblp.xml”)//year/text()

return $years

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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 Summary

Very flexible and powerful language for XML Clean and orthogonal: can always replace a

collection with an expression that creates collections

DB and document-oriented (we hope) The core is relatively clean and easy to

understand

Actually Turing Complete – we’ll talk more about XQuery functions soon

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XSL(T): The Bridge Back to HTML

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

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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

You may want to use XSL/XSLT for your projects – see www.w3.org/TR/xslt for the spec

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Wrapping Up XQuery

We’ve seen three XML manipulation formalisms today: XPath: the basic language for “projecting and

selecting” (evaluating path expressions and predicates) over XML

XQuery: a statically typed, Turing-complete XML processing language

XSLT: a template-based language for transforming XML documents

Each is extremely useful for certain applications!

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Views A view is a named query We can use the name of the view to invoke the query

SQL:CREATE VIEW S(A,B,C) AS

SELECT A,B,C FROM R WHERE R.A = “123”

XQuery:declare function S() as element(content)* {

for $r in doc(“R”)/root, $a in $r/a, $b in $r/b, $c in $r/cwhere $a = “123”return <content>{$a, $b, $c}</content>

}

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What’s Useful about Views

Providing security/access control We can assign users permissions to see only particular views

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) This will be incredibly useful in data integration, discussed

soon

Can be materialized i.e., the results of the view can be saved on disk (as in

caching) Techniques exist for using materialized views to answer other

queries

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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 the results of a view when the base relations change

We can try to do the same thing in reverse 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)

A B

1 2

2 2

B C

2 4

2 3

R S A B C

1 2 4

1 2 3

2 2 4

2 2 3

R⋈S

delete?

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Reasoning about Queries and Views

Given that views have so many uses, it’s helpful to know how and why they’re so useful

First we need a clean way of describing views (let’s assume a relational model for now) Should be declarative Should be less complex than SQL Doesn’t need to support all of SQL – aggregation,

for instance, may be more than we need

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Datalog and Datalog Rules Relational calculus was a first order logic-based way

of expressing queries, where our formulas were: = R(v1, …, vn) | vi = vj | vi = c | Æ ’ | Ç ’ |

8 x. | 9 x. Recall that relational calculus (and first-order logic)

was relationally complete but couldn’t express some operations like transitive closure of graph edges in a relation

Datalog is based on FO logic and “Horn rules”: head(distinguished variables) :- subgoal(variables) & … &

conditions

view(v1, …, vn) :- r1(v1, …, vm) & r2(v3, v4, …, vn) & v1 = “x”

A single Datalog rule, which has no “Ç,” “:,” “8” can express select, project, and join – a conjunctive query

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To Come…

How we answer Datalog queries Reasoning about whether Datalog rules are

“safe”

Comparing Datalog rules to figure out when a view has enough info to answer a different query

Datalog for mapping between schemas … and much, much more!

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Midterm Overview

I expect you to be able to: Apply RA and DRC to answer queries Write simple SQL queries, including joins and aggregation Write simple XQueries, including nesting of output Be able to draw meaningful ER diagrams Tell if something’s in 3NF or BCNF, given FDs Decompose relations using the 3NF algorithm, given a

cover Given a set of FDs, be able to compute a cover

You won’t need to: Compute a closure Apply the BCNF algorithm Write 10-page SQL queries