querying xml: xpath, xquery, and xslt zachary g. ives university of pennsylvania cis 550 –...
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Querying XML: XPath, XQuery, and XSLT
Zachary G. IvesUniversity of Pennsylvania
CIS 550 – Database & Information Systems
October 27, 2005
Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan
2
Reminders
Homework 4 due 11/3 XQuery
Project plan due 11/3 Milestones Division of responsibilities For non-RSS projects: proposal including
scope, milestones, and what you plan to demonstrate
Recitation: Friday 11:30-12:30, Levine 512
3
Talks of Interest
Today: Anastassia Ailamaki, CMU, “FATES: Automatically-tuned
Database Storage Management”, IRCS (3401 Walnut) Room 470 @ 3PM
Tomorrow as part of “DB & Information Retrieval Day”: Anastassia Ailamaki, CMU, “StagedDB: Designing Database
Servers for New Hardware Trends”, Wu and Chen @ 11AM Sam Madden, MIT, “Data Management for Next Generation
Wireless Sensor Networks”, Wu and Chen @ 12:30PM Andrei Broder, Yahoo!, “The next stage in Web IR: From
query based Information Retrieval to context driven Information Supply ”, Wu and Chen @ 2:30PM
4
Querying XML
How do you query a directed graph? a tree?
The standard approach used by many XML, semistructured-data, and object query languages: Define some sort of a template describing
traversals from the root of the directed graph In XML, the basis of this template is called an
XPath
5
XPaths
In its simplest form, an XPath is like a path in a file system:/mypath/subpath/*/morepath
The XPath returns a node set representing the XML nodes (and their subtrees) at the end of the path
XPaths can have node tests at the end, returning only particular node types, e.g., text(), processing-instruction(), comment(), element(), attribute()
XPath is fundamentally an ordered language: it can query in order-aware fashion, and it returns nodes in order
6
Sample XML<?xml version="1.0" encoding="ISO-8859-1" ?> <dblp> <mastersthesis mdate="2002-01-03" key="ms/Brown92"> <author>Kurt P. Brown</author> <title>PRPL: A Database Workload Specification Language</title> <year>1992</year> <school>Univ. of Wisconsin-Madison</school> </mastersthesis> <article mdate="2002-01-03" key="tr/dec/SRC1997-018"> <editor>Paul R. McJones</editor> <title>The 1995 SQL Reunion</title> <journal>Digital System Research Center Report</journal> <volume>SRC1997-018</volume> <year>1997</year> <ee>db/labs/dec/SRC1997-018.html</ee> <ee>http://www.mcjones.org/System_R/SQL_Reunion_95/</ee> </article>
7
XML Data Model VisualizedRoot
?xml dblp
mastersthesis article
mdate key
author title year school editor title yearjournal volume eeee
mdatekey
2002…
ms/Brown92
Kurt P….
PRPL…
1992
Univ….
2002…
tr/dec/…
Paul R.
The…
Digital…
SRC…
1997
db/labs/dec
http://www.
attributeroot
p-i element
text
9
Context Nodes and Relative Paths
XPath has a notion of a context node: it’s analogous to a current directory “.” represents this context node “..” represents the parent node We can express relative paths:
subpath/sub-subpath/../.. gets us back to the context node
By default, the document root is the context node
10
Predicates – Selection Operations
A predicate allows us to filter the node set based on selection-like conditions over sub-XPaths:
/dblp/article[title = “Paper1”]
which is equivalent to:
/dblp/article[./title/text() = “Paper1”]
11
Axes: More Complex Traversals
Thus far, we’ve seen XPath expressions that go down the tree (and up one step) But we might want to go up, left, right, etc. These are expressed with so-called axes:
self::path-step child::path-step parent::path-step descendant::path-step ancestor::path-step descendant-or-self::path-step ancestor-or-self::path-
step preceding-sibling::path-step following-sibling::path-step preceding::path-step following::path-step
The previous XPaths we saw were in “abbreviated form”
12
Querying Order
We saw in the previous slide that we could query for preceding or following siblings or nodes
We can also query a node for its position according to some index: fn:first() , fn:last() return index of 0th & last
element matching the last step: fn:position() gives the relative count of the
current node
child::article[fn:position() = fn:last()]
13
Users of XPath
XML Schema uses simple XPaths in defining keys and uniqueness constraints
XQuery XSLT XLink and XPointer, hyperlinks for XML
14
XQuery
A strongly-typed, Turing-complete XML manipulation language Attempts to do static typechecking against XML Schema Based on an object model derived from Schema
Unlike SQL, fully compositional, highly orthogonal: Inputs & outputs collections (sequences or bags) of XML
nodes Anywhere a particular type of object may be used, may use
the results of a query of the same type Designed mostly by DB and functional language people
Attempts to satisfy the needs of data management and document management The database-style core is mostly complete (even has
support for NULLs in XML!!) The document keyword querying features are still in the
works – shows in the order-preserving default model
15
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}
16
“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
17
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 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>
18
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 document(“dblp.xml”)/universitieswhere $u/country = “USA”return <ms-theses-99>
{ $u/title } { for $mt in $u/../mastersthesis where $mt/year/text() = “1999” and
____________ return $mt/title }
</ms-theses-99>
19
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 $allpapers := document(“dblp.xml”)/dblp/articlereturn <article-authors>
<count> { fn:count(fn:distinct-values($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>
20
Collections, Ctd.
Unlike in SQL, we can compose aggregations and create new collections from old:
<result> {let $avgItemsSold := fn:avg(
for $order in document(“my.xml”)/orders/orderlet $totalSold = fn:sum($order/item/quantity)return $totalSold)return $avgItemsSold
} </result>
21
Distinct-ness
In XQuery, 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
22
Sorting in XQuery
SQL actually allows you to sort its output, with a special ORDER BY clause (which we haven’t discussed, but which specifies a sort key list)
XQuery borrows this idea In XQuery, what we order is the sequence of
“result tuples” output by the return clause:
for $x in document(“dblp.xml”)/proceedingsorder by $x/title/text()return $x
23
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) …}
24
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 }
}
25
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
Turing Complete – we’ll talk more about XQuery functions soon
26
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
27
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)
28
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>
29
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>
30
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
31
Querying XML
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!
32
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/cwhere $a = “123”return <content>{$a, $b, $c}</content>
}
SELECT * FROM V, RWHERE V.B = 5 AND V.C = R.C
for $v in V()/content, $r in doc(“r”)/root/treewhere $v/b = $r/breturn $v
Using the views:
33
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
34
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, RWHERE V.B = 5 AND V.C = R.C
35
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)
36
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!
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?