XML,%20Schemas,%20and%20XPath

1
XML, Schemas, and XPath

• Zachary G. Ives
• University of Pennsylvania
• CIS 550 Database Information Systems
• October 14, 2004

Some slide content courtesy of Susan Davidson
Raghu Ramakrishnan
2
Announcements

• Homework 3 due today
• Homework 4 handed out
• Midterm Thursday 10/28

3
Why Were Interested in XML

• Can get data from all sorts of sources
• Allows us to touch data we dont own!
• This was actually a huge change in the DB
  community
• Used for sharing data
• 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!

4
Basic XML Anatomy
Processing Instr.

• lt?xml version"1.0" encoding"ISO-8859-1" ?gt
• ltdblpgt
• ltmastersthesis mdate"2002-01-03"
  key"ms/Brown92"gt
•   ltauthorgtKurt P. Brownlt/authorgt
•   lttitlegtPRPL A Database Workload
  Specification Languagelt/titlegt
•   ltyeargt1992lt/yeargt
•   ltschoolgtUniv. of Wisconsin-Madisonlt/schoolgt
•   lt/mastersthesisgt
• ltarticle mdate"2002-01-03" key"tr/dec/SRC1997-
  018"gt
•   lteditorgtPaul R. McJoneslt/editorgt
•   lttitlegtThe 1995 SQL Reunionlt/titlegt
•   ltjournalgtDigital System Research Center
  Reportlt/journalgt
•   ltvolumegtSRC1997-018lt/volumegt
•   ltyeargt1997lt/yeargt
•   lteegtdb/labs/dec/SRC1997-018.htmllt/eegt
•   lteegthttp//www.mcjones.org/System_R/SQL_Reunio
  n_95/lt/eegt
•   lt/articlegt

Open-tag
Element
Attribute
Close-tag
5
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
• lttag/gt shortcut for empty tags (equivalent to
  lttaggtlt/taggt
• Attributes (which are unordered, in contrast to
  elements) only appear once in an element
• Theres a single root element
• XML is case-sensitive

6
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

7
XML Data Model Visualized(and simplified!)
attribute
root
p-i
element
Root
text
dblp
?xml
mastersthesis
article
mdate
mdate
key
key
author
title
year
school
2002
editor
title
year
journal
volume
ee
ee
2002
1992
1997
The
ms/Brown92
tr/dec/
PRPL
Digital
db/labs/dec
Univ.
Paul R.
Kurt P.
SRC
http//www.
8
What Does XML Do?

• Serves as a document format (super-HTML)
• Allows custom tags (e.g., used by MS Word,
  openoffice)
• Supplement it with stylesheets (XSL) to define
  formatting
• Data exchange format (must agree on terminology)
• Marshalling and unmarshalling data in SOAP and
  Web Services

9
XML as a Super-HTML(MS Word)

• lth1 class"Section1"gtlta name"_top /gtCIS 550
  Database and Information Systemslt/h1gt
• lth2 class"Section1"gtFall 2004lt/h2gt
• ltp class"MsoNormal"gt
• ltplacegt311 Townelt/placegt, Tuesday/Thursday
• lttime Hour"13" Minute"30"gt130PM
  300PMlt/timegt
• lt/pgt

10
XML Easily Encodes Relations
Student-course-grade
sid serno exp-grade
1 570103 B
23 550103 A

• ltstudent-course-gradegt
• lttuplegtltsidgt1lt/sidgtltsernogt570103lt/sernogtltexp-grad
  egtBlt/exp-gradegtlt/tuplegt
• lttuplegtltsidgt23lt/sidgtltsernogt550103lt/sernogtltexp-gra
  degtAlt/exp-gradegtlt/tuplegt
• lt/student-course-gradegt

11
But XML is More FlexibleNon-First-Normal-Form
(NF2)

• ltparentsgt
• ltparent nameJean gt
• ltsongtJohnlt/songt
• ltdaughtergtJoanlt/daughtergt
• ltdaughtergtJilllt/daughtergt
• lt/parentgt
• ltparent nameFenggt
• ltdaughtergtFelicitylt/daughtergt
• lt/parentgt

Coincides with semi-structured data, invented
by DB people at Penn and Stanford
12
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

13
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
• ltroot xmlnshttp//www.first.com/aspace
  xmlnsothernsgt
• lttag xmlnsmynshttp//www.fictitious.com/mypath
  gt
• ltthistaggtis in the default namespace
  (aspace)lt/thistaggt
• ltmynsthistaggtis in mynslt/mynsthistaggtltotherns
  thistaggtis a different tag in othernslt/othernsthi
  staggt
• lt/taggt
• lt/rootgt

14
XML Isnt Enough on Its Own

• Its too unconstrained for many cases!
• How will we know when were 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)
• Well talk about this soon
• Some way of interpreting the tags?
• Well talk about this later in the semester

15
Structural ConstraintsDocument 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

16
An Example DTD

• Example DTD
• lt!ELEMENT dblp((mastersthesis article))gt
• lt!ELEMENT mastersthesis(author,title,year,school,c
  ommitteemember)gt
• lt!ATTLIST mastersthesis(mdate CDATA REQUIRED ke
  y ID REQUIRED
• advisor CDATA IMPLIEDgt
• lt!ELEMENT author(PCDATA)gt
• Example use of DTD in XML file
• lt?xml version"1.0" encoding"ISO-8859-1" ?gt
• lt!DOCTYPE dblp SYSTEM my.dtd"gt
• ltdblpgt

17
Representing Graphs and Links in XML

• lt?xml version"1.0" encoding"ISO-8859-1" ?gt
• lt!DOCTYPE graph SYSTEM special.dtd"gt
• ltgraphgt
• ltauthor idauthor1gt
• ltnamegtJohn Smithlt/namegt
• lt/authorgt
• ltarticlegt
• ltauthor refauthor1 /gt lttitlegtPaper1lt/titlegt
• lt/articlegt
• ltarticlegt
• ltauthor refauthor1 /gt lttitlegtPaper2lt/titlegt
• lt/articlegt

18
Graph Data Model
Root
graph
?xml
!DOCTYPE
article
article
author
id
title
title
author
author
name
Paper1
author1
ref
Paper2
ref
John Smith
author1
author1
19
Graph Data Model
Root
graph
?xml
!DOCTYPE
article
article
author
id
title
title
author
author
name
Paper1
author1
ref
Paper2
ref
John Smith
20
DTDs Arent Expressive Enough

• DTDs capture grammatical structure, but have some
  drawbacks
• Not themselves in XML inconvenient to build
  tools for them
• Dont capture database datatypes domains
• IDs arent a good implementation of keys
• Why not?
• No way of defining OO-like inheritance

21
XML Schema

• Aims to address the shortcomings of DTDs
• XML syntax
• Can define keys using XPaths
• Type subclassing thats more complex than in a
  programming language
• Programming languages dont consider order of
  member variables!
• Subclassing by extension and by restriction
• And, of course, domains and built-in datatypes

22
Basics of XML Schema

• Need to use the XML Schema namespace (generally
  named xsd)
• simpleTypes are a way of restricting domains on
  scalars
• Can define a simpleType based on integer, with
  values within a particular range
• complexTypes are a way of defining
  element/attribute structures
• Basically equivalent to !ELEMENT, but more
  powerful
• Specify sequence, choice between child elements
• Specify minOccurs and maxOccurs (default 1)
• Must associate an element/attribute with a
  simpleType, or an element with a complexType

23
Simple Schema Example

• ltxsdschema xmlnsxsd"http//www.w3.org/2001/XMLS
  chema"gt
• ltxsdelement namemastersthesis"
  typeThesisType"/gt
• ltxsdcomplexType nameThesisType"gt
• ltxsdattribute namemdate" type"xsddate"/gt
• ltxsdattribute namekey" type"xsdstring"/gt
• ltxsdattribute nameadvisor" type"xsdstring"/gt
• ltxsdsequencegt
• ltxsdelement nameauthor" typexsdstring"/gt
• ltxsdelement nametitle" typexsdstring"/gt
• ltxsdelement nameyear" typexsdinteger"/gt
• ltxsdelement nameschool" typexsdstring/gt
• ltxsdelement namecommitteemember"
  typeCommitteeType minOccurs0"/gt
• lt/xsdsequencegt
• lt/xsdcomplexTypegt
• lt/xsdschemagt

24
Designing an XML Schema/DTD

• 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!
• Generally orient the XML tree around the
  central objects
• 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!

25
XML as a Data Model

• XML is a non-first-normal-form (NF2)
  representation
• Can represent documents, data
• Standard data exchange format
• Several competing schema formats esp., DTD and
  XML Schema provide typing information
• Next basics of querying XML

26
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

27
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

28
Sample XML

• lt?xml version"1.0" encoding"ISO-8859-1" ?gt
• ltdblpgt
• ltmastersthesis mdate"2002-01-03"
  key"ms/Brown92"gt
•   ltauthorgtKurt P. Brownlt/authorgt
•   lttitlegtPRPL A Database Workload
  Specification Languagelt/titlegt
•   ltyeargt1992lt/yeargt
•   ltschoolgtUniv. of Wisconsin-Madisonlt/schoolgt
•   lt/mastersthesisgt
• ltarticle mdate"2002-01-03" key"tr/dec/SRC1997-
  018"gt
•   lteditorgtPaul R. McJoneslt/editorgt
•   lttitlegtThe 1995 SQL Reunionlt/titlegt
•   ltjournalgtDigital System Research Center
  Reportlt/journalgt
•   ltvolumegtSRC1997-018lt/volumegt
•   ltyeargt1997lt/yeargt
•   lteegtdb/labs/dec/SRC1997-018.htmllt/eegt
•   lteegthttp//www.mcjones.org/System_R/SQL_Reunio
  n_95/lt/eegt
•   lt/articlegt

29
XML Data Model Visualized
attribute
root
p-i
element
Root
text
dblp
?xml
mastersthesis
article
mdate
mdate
key
key
author
title
year
school
2002
editor
title
year
journal
volume
ee
ee
2002
1992
1997
The
ms/Brown92
tr/dec/
PRPL
Digital
db/labs/dec
Univ.
Paul R.
Kurt P.
SRC
http//www.
30
Some Example XPath Queries

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

31
Context Nodes and Relative Paths

• XPath has a notion of a context node its
  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

32
Predicates Selection Operations

• A predicate allows us to filter the node set
  based on selection-like conditions over
  sub-XPaths
• /dblp/articletitle Paper1
• which is equivalent to
• /dblp/article./title/text() Paper1

33
Axes More Complex Traversals

• Thus far, weve 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
• selfpath-step
• childpath-step parentpath-step
• descendantpath-step ancestorpath-step
• descendant-or-selfpath-step ancestor-or-selfpa
  th-step
• preceding-siblingpath-step following-siblingpa
  th-step
• precedingpath-step followingpath-step
• The previous XPaths we saw were in abbreviated
  form

34
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
• fnfirst() , fnlast() return index of 0th
  last element matching the last step
• fnposition() gives the relative count of the
  current node
• childarticlefnposition() fnlast()

35
Users of XPath

• XML Schema uses simple XPaths in defining keys
  and uniqueness constraints
• XQuery
• XSLT
• XLink and XPointer, hyperlinks for XML
• Next time well focus on XQuery, the
  nearly-complete SQL of XML
• And well briefly discuss XSLT, a different
  attempt to manipulate XML data