XML: Semistructured Data

Zachary G. IvesUniversity of Pennsylvania

CIS 550 – Database & Information Systems

October 2, 2003

Some slide content courtesy of Susan Davidson & Raghu Ramakrishnan

2

Administrivia

HW3 due Tuesday 10/7 Please return HW2s for grade recording (sorry…) My office is now 576 GRW

(5th floor of the old CIS bldg., by the old stairwell) Upcoming schedule:

Tues. 10/7: XQuery & XSLT; intro to views; HW4 out (Read Section 3.6 in book)

Thurs. 10/9: Views + datalog, ctd.; review for midterm

Tues. 10/14: Fall Break Thurs. 10/16: HW4 due; Midterm

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Why XML?

XML is the confluence of several factors: The Web needed a more declarative format for data Documents needed a mechanism for extended tags Database people needed a more flexible interchange

format “Lingua franca” of data It’s parsable even if we don’t know what it means!

Original expectation: The whole web would go to XML instead of HTML

Today’s reality: Not so… But XML is used all over “under the covers”

4

Why DB People Like XML

Can get data from all sorts of things Allows us to touch data we don’t own! This was actually a huge change in the DB community

Interesting relationships with DB techniques Useful to do relational-style operations Leverages ideas from object-oriented, semistructured

data

Blends schema and data into one format Unlike relational model, where we need schema first … But too little schema can be a drawback, too!

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XML Anatomy<?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>

Processing Instr.

Element

Attribute

Close-tag

Open-tag

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Well-Formed XML

A legal XML document – fully parsable by an XML parser All open-tags have matching close-tags (unlike

so many HTML documents!), or a special:<tag/> shortcut for empty tags (equivalent to

<tag></tag>

Attributes (which are unordered, in contrast to elements) only appear once in an element

There’s a single root element XML is case-sensitive

7

XML as a Data Model

XML “information set” includes 7 types of nodes: Document (root) Element Attribute Processing instruction Text (content) Namespace: Comment

XML data model includes this, plus typing info, plus order info and a few other things

8

XML Data Model Visualized(and simplified!)

Root

?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

What Does XML Do?

Serves as a document format that’s better than HTML Allows custom tags (e.g., used by MS Word,

openoffice) Supplement it with stylesheets (XSL) to define

formatting Provides an exchange format for data (still

need to agree on terminology) Basis for RDF format for describing libraries

and the Semantic Web (more on this later) Format for marshalling and unmarshalling

data in SOAP and Web Services

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XML as a Super-HTML(MS Word)

<h1 class="Section1"><a name="_top“ />CIS 550: Database and Information Systems</h1><h2 class="Section1">Fall 2003</h2><p class="MsoNormal">

<place>311 Towne</place>, Tuesday/Thursday<time Hour="13" Minute="30">1:30PM –

3:00PM</time></p>

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XML Easily Encodes Relations

sid

serno

exp-grade

1 570103

B

23 550103

A<student-course-grade><tuple><sid>1</sid><serno>570103</serno><exp-grade>B</exp-grade></tuple><tuple><sid>23</sid><serno>550103</serno><exp-grade>A</exp-grade></tuple>

</student-course-grade>

Student-course-grade

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But XML is More Flexible…“Non-First-Normal-Form” (NF2)

<parents> <parent name=“Jean” >

<son>John</son><daughter>Joan</daughter><daughter>Jill</daughter>

</parent> <parent name=“Feng”>

<daughter>Felicity</daughter> </parent>…

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XML and Code

Web Services (.NET, recent Java web service toolkits) are using XML to pass parameters and make function calls Why?

Easy to be forwards-compatible Easy to read over and validate (?) Generally firewall-compatible

Drawbacks? XML is a verbose and inefficient encoding!

XML is used to represent: SOAP: the “envelope” that data is marshalled into XML Schema: gives some typing info about structures being

passed WSDL: the IDL (interface def language) UDDI: provides an interface for querying about web services

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Integrating XML: What If We Have Multiple Sources with the Same Tags?

Namespaces allow us to specify a context for different tags

Two parts: Binding of namespace to URI Qualified names

<tag xmlns:myns=“http://www.fictitious.com/mypath”><thistag>is in namespace myns</thistag><myns:thistag>is the same</myns:thistag><otherns:thistag>is a different tag</otherns:thistag>

</tag>

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XML Isn’t Enough on Its Own

It’s too unconstrained for many cases! How will we know when we’re getting garbage? How will we query? How will we understand what we got?

We also need: Some idea of the structure

Our focus next Presentation, in some cases – XSL(T)

We’ll talk about this soon Some way of interpreting the tags…?

We’ll talk about this later in the semester

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Document Type Definitions (DTDs)

The DTD is an EBNF grammar defining XML structure XML document specifies an associated DTD, plus

the root element DTD specifies children of the root (and so on)

DTD defines special significance for attributes: IDs – special attributes that are analogous to keys

for elements IDREFs – references to IDs IDREFS – a nasty hack that represents a list of

IDREFs

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An Example DTD

Example DTD:<!ELEMENT dblp((mastersthesis | article)*)><!ELEMENT mastersthesis(author,title,year,school,committeemember*)><!ATTLIST mastersthesis(mdate CDATA #REQUIRED

key ID #REQUIREDadvisor CDATA #IMPLIED>

<!ELEMENT author(#PCDATA)>

…Example use of DTD in XML file:

<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE dblp SYSTEM “my.dtd"> <dblp>…

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Representing Graphs in XML

<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE graph SYSTEM “special.dtd"> <graph>

<author id=“author1”><name>John Smith</name>

</author><article>

<author ref=“author1” /> <title>Paper1</title></article><article>

<author ref=“author1” /> <title>Paper2</title></article>

…

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Graph Data ModelRoot

!DOCTYPE

graph

authorarticle

nametitle

refref

John Smith

author1author1

Paper2

?xml

article

id

author1

author authortitle

Paper1

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Graph Data ModelRoot

!DOCTYPE

graph

authorarticle

nametitle

refref

John Smith

Paper2

?xml

article

id

author1

author authortitle

Paper1

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DTDs Aren’t Enough for Some People

DTDs capture grammatical structure, but have some drawbacks: Not themselves in XML – inconvenient to build

tools for them Don’t capture database datatypes’ domains IDs aren’t a good implementation of keys

Why not?

No way of defining OO-like inheritance No way of expressing FDs

22

XML Schema

Aims to address the shortcomings of DTDs… But an “everything including the kitchen sink” spec

Features: XML syntax Better way of defining keys using XPaths Type subclassing that’s more complex than in a

programming language Programming languages don’t consider order of member

variables! Subclassing “by extension” and “by restriction”

… And, of course, domains and built-in datatypes

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Simple Schema Example

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<xsd:element name=“mastersthesis" type=“ThesisType"/> <xsd:complexType name=“ThesisType">

<xsd:attribute name=“mdate" type="xsd:date"/><xsd:attribute name=“key" type="xsd:string"/><xsd:attribute name=“advisor" type="xsd:string"/><xsd:sequence>

<xsd:element name=“author" type=“xsd:string"/> <xsd:element name=“title" type=“xsd:string"/> <xsd:element name=“year" type=“xsd:integer"/> <xsd:element name=“school" type=“xsd:string”/> <xsd:element name=“committeemember"

type=“CommitteeType” minOccurs=“0"/> </xsd:sequence>

</xsd:complexType>

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Whither XML Schema?

Adopted by a few tools, but… The jury is still out on this spec

Considered too big, bulky, inconsistent Still no way of expressing FDs, and the key

mechanism isn’t that elegant Other people have defined more elegant schema

languages, FD languages, and key languages (much of the work was here at Penn)

But it’s also at the core of the data model of the XQuery language and the other W3C specs…

25

Designing an XML Schema/DTD

XML data design is generally not as formalized as relational data design We can still use ER diagrams to break into entity,

relationship sets ER diagrams have extensions for “aggregation” – treating

smaller diagrams as entities – and for composite attributes Note that often we already have our data in relations and

need to design the XML schema to export them! We generally orient the XML tree around the

“central” objects in a particular application A big decision: element vs. attribute

Element if it has its own properties, or if you *might* have more than one of them

Attribute if it is a single property – or perhaps not!

26

XML as a Data Model

XML is a non-first-normal-form representation Can represent documents, data Standard data exchange format Several competing schema formats – esp.,

DTD and XML Schema – provide typing information

Now: the core ideas in querying XML

27

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

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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

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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>

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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

31

Some Example XPath Queries

/dblp/mastersthesis/title /dblp/*/editor //title //title/text()

32

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

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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”]

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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”

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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()]

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Users of XPath

XML Schema uses simple XPaths in defining keys and uniqueness constraints

XQuery XSLT XLink and XPointer, hyperlinks for XML

Next time we’ll focus on XQuery, the still-under-development “SQL of XML”…

… And we’ll briefly discuss XSLT, a different attempt to manipulate XML data