1 Lecture 08: XML and Semistructured Data

2 Outline XML (Section 17) –XML syntax, semistructured data –Document Type Definitions (DTDs) XPath

3 Additional Readings on XML XML – – – – (1/99) Xpath – Xquery – – Main source: (but hard to read)

4 XML eXtensible Markup Language XML 1.0 – a recommendation from W3C, 1998 Roots: SGML (a very nasty language). After the roots: a format for sharing data

5 XML Data Relational data does not have a syntax –I can’t “give” you my relational database –Need to import it from other syntax, like CSV (comma-separated-values) XML = rich syntax for data –But XML is not relational: semistructured Usage: –Map any data to XML –Store it in files, exchange on the Web, etc. –Even query it directly, using XPath, XQuery

6 XML Data Sharing and Exchange application relational data Transform Integrate Warehouse XML DataWEB (HTTP) application legacy data object-relational Specific data management tasks

7 From HTML to XML HTML describes the presentation

8 HTML Bibliography Foundations of Databases Abiteboul, Hull, Vianu Addison Wesley, 1995 Data on the Web Abiteoul, Buneman, Suciu Morgan Kaufmann, 1999 Bibliography Foundations of Databases Abiteboul, Hull, Vianu Addison Wesley, 1995 Data on the Web Abiteoul, Buneman, Suciu Morgan Kaufmann, 1999

9 XML Foundations… Abiteboul Hull Vianu Addison Wesley 1995 … Foundations… Abiteboul Hull Vianu Addison Wesley 1995 … XML describes the content

10 XML Terminology tags: book, title, author, … start tag:, end tag: elements: …, … elements are nested empty element: abbrv. an XML document: single root element well formed XML document: if it has matching tags

11 More XML: Attributes Foundations of Databases Abiteboul … 1995 Foundations of Databases Abiteboul … 1995 attributes are alternative ways to represent data

12 More XML: IDs and References Jane Mary John Jane Mary John Scope of IDs and references is the document

13 More XML: CDATA Section Syntax: Example: <>]]>

14 More XML: Entity References Syntax: &entityname; Example: this is less than < Some entities: << >> && &apos;‘ "“ &Unicode char

15 More XML: Processing Instructions Syntax: Example: Processed by external applications, e.g. php (bad style) Alarm Clock 19.99

16 More XML: Comments Syntax Yes, they are part of the data model !!!

17 XML Namespaces XML namespace is a collection of names (markup vocabulary) –identified by a prefix (URL reference) name ::= [prefix:]localname <book xmlns='urn:loc.gov:book' xmlns:isbn=' … 15 …. <book xmlns='urn:loc.gov:book' xmlns:isbn=' … 15 …. default name space names belong to default name space

18 … … XML Namespaces syntactic:, semantic: URL used as unique identifier –URL may not exist, has no function Belong to this namespace

19 XML Data: a Tree ! Mary Maple 345 Seattle John Thailand Mary Maple 345 Seattle John Thailand data Mary person name address name address streetnocity Maple345 Seattle John Thai phone id o555 Element node Text node Attribute node Order matters !!!

20 From Relational Data to XML Data John 3634 Sue 6343 Dick 6363 John 3634 Sue 6343 Dick 6363 row name phone “John”3634“Sue”“Dick” persons XML: persons

21 XML Data XML is self-describing Schema elements become part of the data –Relational schema: persons(name,phone) –In XML,, are part of the data, and are repeated many times Consequence: XML is much more flexible XML = semistructured data

22 Semi-structured Data Explained Missing attributes: Could represent in a table with nulls John 1234 Joe John 1234 Joe  no phone ! namephone John1234 Joe-

23 Semi-structured Data Explained Repeated attributes Impossible in tables: Mary Mary  two phones ! namephone Mary ???

24 Semistructured Data Explained Attributes with different types in different objects Nested collections (no 1NF) Heterogeneous collections: – contains both s and s John Smith 1234 John Smith 1234  structured name !

25 Document Type Definitions DTD part of the original XML specification an XML document may have a DTD XML document: well-formed = if tags are correctly closed Valid = if it has a DTD and conforms to it validation is useful in data exchange

26 Very Simple DTD <!DOCTYPE company [ ]> <!DOCTYPE company [ ]>

27 Very Simple DTD John B Jim B John B Jim B Example of valid XML document:

28 DTD: The Content Model Content model: –Complex = a regular expression over other elements –Text-only = #PCDATA –Empty = EMPTY –Any = ANY –Mixed content = (#PCDATA | A | B | C)* content model

29 DTD: Regular Expressions <!ELEMENT name (firstName, lastName)) <!ELEMENT name (firstName?, lastName)) DTDXML <!ELEMENT person (name, phone*)) sequence optional <!ELEMENT person (name, (phone| ))) star (repeated occurrence) alternation

30 DTD: Attributes Document Type Definition Document … … … mandatory optional default enumeration

31 Inclusion of DTD in Documents "test" is a document element ]> &hello; ]> External DTD Declaration Internal DTD Declaration Mixed usage

32 Querying XML Data XPath = simple navigation through the tree XQuery = the SQL of XML XSLT = recursive traversal –will not discuss XQuery and XSLT build on XPath

33 Sample Data for Queries Addison-Wesley Serge Abiteboul Rick Hull Victor Vianu Foundations of Databases 1995 Freeman Jeffrey D. Ullman Principles of Database and Knowledge Base Systems 1998 Addison-Wesley Serge Abiteboul Rick Hull Victor Vianu Foundations of Databases 1995 Freeman Jeffrey D. Ullman Principles of Database and Knowledge Base Systems 1998

34 Data Model for XPath bib book publisherauthor.. Addison-WesleySerge Abiteboul The root The root element

35 XPath: Simple Expressions Result: Result: empty (there were no papers) /bib/book/year /bib/paper/year

36 XPath: Restricted Kleene Closure Result: Serge Abiteboul Rick Hull Victor Vianu Jeffrey D. Ullman Result: Rick //author /bib//first-name

37 XPath: Text Nodes Result: Serge Abiteboul Jeffrey D. Ullman Rick Hull doesn’t appear because he has firstname, lastname Functions in XPath: –text() = matches the text value –node() = matches any node (= * or text()) –name() = returns the name of the current tag /bib/book/author/text()

38 XPath: Wildcard Result: Rick Hull * Matches any element //author/*

39 XPath: Attribute Nodes Result: means that price is has to be an attribute

40 XPath: Predicates Result: Rick Hull /bib/book/author[firstname]

41 XPath: More Predicates Result: … … /bib/book/author[firstname][address[//zip][city]]/lastname

42 XPath: More Predicates < “60”] < “25”] /bib/book[author/text()]

43 XPath: Summary bibmatches a bib element *matches any element /matches the root element /bibmatches a bib element under root bib/papermatches a paper in bib bib//papermatches a paper in bib, at any depth //papermatches a paper at any depth paper|bookmatches a paper or a a price attribute price attribute in book, in bib matches…